Zurich Open Repository and Archive University of Zurich Main Library Strickhofstrasse 39 CH-8057 Zurich www.zora.uzh.ch
Year: 2013
Homogeneous hydrogenations and related reductive reactions catalyzed by rhenium complexes
Kunjanpillai, Rajesh
Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-90938 Dissertation Published Version
Originally published at: Kunjanpillai, Rajesh. Homogeneous hydrogenations and related reductive reactions catalyzed by rhenium complexes. 2013, University of Zurich, Faculty of Science. Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
DISSERTATION
zur Erlangung der naturwissenschaftlichen Doktorwürde (Dr. sc. nat.) vorgelegt der Mathematisch-naturwissenschaftlichen Fakultät der Universität Zürich
von Rajesh Kunjanpillai aus Indien
Promotionskomitee Prof. Dr. Heinz Berke (Vorsitz und Leitung) Prof. Dr. Roger Alberto
Zürich 2013
Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
DISSERTATION
zur Erlangung der naturwissenschaftlichen Doktorwürde (Dr. sc. nat.) vorgelegt der Mathematisch-naturwissenschaftlichen Fakultät der Universität Zürich
von Rajesh Kunjanpillai aus Indien
Promotionskomitee Prof. Dr. Heinz Berke (Vorsitz und Leitung) Prof. Dr. Roger Alberto
Zürich 2013
Dedicated to my father
N. Kunjan Pillai (1935-1993)
List of Abbreviations
List of Abbreviations
Acronym Full Name
TOF Turn Over Frequency
TON Turn Over Number
THF Tetrahydrofuran
TMP 2,2,6,6-Tetramethylpiperidine
DMF N,N-Dimethylformamide p-TsOH p-Toluenesulfonic acid equiv. equivalents
KIE Kinetic Isotopic Effect
Me methyl
Et ethyl
Pr propyl
Bu butyl
Ph phenyl o ortho m meta p para n normal i iso t tertiary cy cyclo Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
h hour
IR Infrared
ν frequency
vs versus
NMR Nuclear Magnetic Spectroscopy
δ chemical shift
ppm parts per million
Hz Hertz
s singlet
d doublet
t triplet
q quartet
quin quintet
GC/MS Gas Chromatography/Mass Spectroscopy
min minute
∆Go standard Gibbs free energy change
∆Ho standard enthalpy change
∆So standard entropy change
kJ kilo Joules
K Kelvin
°C degree Celsius
g gaseous
aq aqueous
eq equation Table of Contents
Table of Contents
1 General Introduction...... 1-30 1.1. Significance of Catalysis...... 1 1.2. Hydrogen and its Activation...... 1 1.3. Homogeneous Hydrogenation...... 3 1.4. Hydrogenation of various Functional Groups……………………………………..4 1.4.1. Hydrogenation of Alkenes…………………………...... 4 1.4.2. Hydrogenation of Aldehydes and Ketones...... 9 1.4.2.1. Aldehydes………………………………………………………….……...9 1.4.2.2. Ketones…………………………………………………………………..10 1.4.3. Reductive Amination and Hydrogenation of Imines...…………………….12 1.4.4. Hydrogenation of Nitriles………………...………………………………..14 1.4.5. Hydrogenation of Carboxylic Esters…………...………………………….14 1.4.6. Hydrogenation of Carbon Dioxide and Carbonates/Bicarbonates……….. 16 1.5. Transfer Hydrogenation...... 17 1.5.1. Transfer Hydrogenation of Ketones and Imines...... 18 1.5.2. Transfer Hydrogenation of Nitriles……...... 20 1.6. Hydrosilylation of Nitriles………………………………………….……………20 1.7. Claisen-Tishchenko Reaction of Aldehydes…………...... ……....21 1.8. Goal of the Project………………………………………………………………..22 1.9. References…………………………………………………………………….…..24
2 Large Bite-Angle Diphosphine Nitrosyl Rhenium Complexes as Highly
Efficient Catalysts for Olefin Hydrogenations…………………………………….31-61 2.1 Introduction ………………………………………………………….…………..31 2.2. Results and Discussion……………………………………………………………...33 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
2.2.1. Preparation of [Re(diphosphine)Br2(NO)(CH3CN)] (IIA-IIC) and
[Re2(Sixantphos)2(Br)2(µ-Br)2(NO)2].2CH3CN (IIIA)………………...….33
2.2.2. Reaction of IIA-IIB and IIIA with Et3SiH……………………...….……..38 2 2.2.3. Preparation of [Re(oCPPh-P∩P)(η -ethylene)Br(NO)] (VII)……...………39 2.2.4. Hydrogenation of Olefins using the Complexes of the Type VII……...….42 2.2.5. Kinetics and Mechanism of the Hydrogenations Using Complexes of the Type VII…………………………………….………………………45 2.2.6. Hydrogenation of Olefins and their Mechanism Using Complexes of the Type II and IIIA………………………………….…………………47
2.2.7. Preparation of [Re(POP)I2(NO)] (POP = A, XIIIA) and Catalytic Hydrogenation of Styrene………………………..50
2.2.8. Preparation of [Re(DBFmonophos)(CH3CN)2Br2(NO)] (XIVD) and Catalytic Hydrogenation of Styrene…………………………………52 2.3. Conclusion…………………………………………………………………………..53 2.4. Experimental Section………………………………………………………………..54 2.5. References……………………………………………………...…………………….60
3 Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium Complexes…62-73 3.1 Introduction...... 62 3.2 Results and discussion...... 64 3.2.1. Hydrogenation of Nitriles Catalyzed by Complexs of the Type II,
III and VII………………………………………………………………………..64
3.2.3. Mechanistic Studies………………………………………………………..68 3.3. Conclusion …………………………………………………………………………..71 3.4. Experimental Section………………………………………………………...………71 3.5. References……………………………………………………..……………………..72
4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on reversible Halide Dissociation………………………………………………………………….74-99
Table of Contents
4.1 Introduction...... 74 4.2. Results and Discussion……………………………………………………………..76 4.2.1. Reductive Amination of Aldehydes Using Complexes of the Type II,
III and VII………………………...…………………………………………...…76
4.2.2. Hydrogenation of Imines Using Complexes of the Type IIIA and
XIIIA………………………………………………………………………83
4.2.3. Mechanistic Studies and Preparation of [Re(A)(Br)3(NO)][NEt4]
(XIXA) and formation of [Re(POP)(Br)2(NO)] (POP = A)… …………....87
4.3. Conclusion………………………………………………………………………..….94
4.4. Experimental Section………………………………………………………………...95
4.5. References……………………………………………………………………...…….9
5. Homogeneous Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol Catalyzed by Rhenium Complexes……………………..………….....100-112 5.1. Introduction……………………………………………………………………...…100 5.2. Results and Discussion……………………………………………………………..101
5.2.1. Hydrogenation of CO2 to MeOH Catalyzed by Complexes IIIA, XVIIIA, XIXA and XXVIA, as well as the preparation of 3 [Re2(A)2µ -(OH)3(NO)2][Br] (XXVIA)………………………………….101 5.2.2. Hydrosilylation and Combined Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol catalyzed by Complexes IIIA and XVIIA……………………………………………………………………105 5.2.3. Mechanistic Studies……………………………………………………....108 5.3. Conclusion……………………………………………………………………….....109 5.4. Experimental Section……………………………………………………………….110 5.5. References…………………………………………………………………………..111
6. Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Formates Catalyzed by Rhenium Complexes…………………………………...113-127 6.1. Introduction…………………………………………………………………...……113 6.2. Results and Discussion……………………………………………………………..114 6.2.1. Hydrogenation of Aldehydes Catalyzed by IIIA………………………...114 6.2.2. Hydrogenation of Ketones catalyzed by IIIA…………………………....118 6.2.3. Hydrogenation of Esters and Bicarbonates to Alcohols Catalyzed by IIIA………………………………………………………………...….119
6.2.4. Hydrogenation of CO2 and bicarbonates to formates Catalyzed by IIIA……………………………………………………………………….120 6.2.5. Mechanistic Aspects……………………………………………………...121 6.3. Conclusion………………………………………………………………………….124 6.4. Experimental Section……………………………………………………………….125 6.5. References………………………………………………………………………... 126
7. Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer
Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes…………………………………………………………………………128-150 7.1 Introduction…………………………………………………………………………128 7.2. Claisen-Tishchenko Reaction of Aldehydes Catalyzed by IIIA…………………...129 7.2.1. Results and Discussion………………………………………………...…129 7.2. 2. Mechanistic Studies……………………………………………………...134 7.3. Transfer Hydrogenations of Ketones and Imines catalysed by IIIA…………...…..136 7.3.1 Transfer Hydrogenation of Ketones………………………………………137 7.3.1.1. Results and Discussion………………………………………………....137 7.3.2. Transfer Hydrogenation of Imines………………………………………..142 7.3.2.1. Results and Discussion…………………………………………………142 7.3.4. Mechanistic Studies………………………………………………………144 7.4. Conclusion……………………………………………………………………….…147 7.5. Experimental Section……………………………………………………………….147 7.6. References…………………………………………………………………………..148
Table of Contents
8. Homogeneous Thermocontrolled Chemoselective Transfer Hydrogenations of Nitriles Catalyzed by Rhenium Complexes……………...……………….…..151-159 8.1. Introduction……………………………………………………………………...…151 8.2. Results and discussion………………………………………………………….…..152 8.2.1. Transfer Hydrogenation of Nitriles Catalyzed by IIIA………………..…152 8.2. 2. Mechanistic Aspects……………………………………………….….…156 8.3. Conclusion………………………………………………………………………….157 8.4. Experimental Section……………………………………………………………….157 8.5. References…………………………………………………………………………..158
9. Homogeneous Hydrosilylations of Nitriles Catalyzed by Rhenium Complexes
……………………………………………………………………………………...160-167 9.1. Introduction………………………………………………………………………...160 9.2. Results and Discussion……………………………………………………………..160 9.2.1. Hydrosilylation of Nitriles Catalyzed by IIIA…………………………...160 9.2.2. Mechanistic Aspects…………………………………………………..….164 9.3. Conclusion………………………………………………………………………….165 9.4. Experimental Section…………………………………………………………….…166 9.5. References……………………………………………………………………….…166
10 Alkali Metal tert-Butoxides, Hydrides and Bis(trimethylsilyl)amides as Efficient Homogeneous Catalysts for Claisen-Tishchenko Reaction...... 168-196
11. Summary……………………………………………………………………..……197-213
12. List of New Compounds………………………………………………...……………..214
13. Abstract……………………………………………………………………..………….215
14. Zusammenfassung…………………………………………………………………...... 216
15 Acknowledgements………………………...……………………………...………217-218
16 Curriculum Vitae………………………………………………………………....219-222
Chapter 1 General Introduction
1. General Introduction
1.1. Significance of Catalysis
Efficient and environmentally friendly transformations are challenging goals in chemical synthesis. The greatest achievements in these respects have evoked from the concept of
‘catalysis’. A catalyst is any substance that increases the rate of a reaction without itself being consumed; and the process of application of it can be termed as catalysis. About 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture.1 It is extensively applied in energy processing, production of bulk, fine and intermediate chemicals etc. The two broad classification of catalysis are homogeneous and heterogeneous catalysis. In the homogeneous catalysis, the catalyst and the reactants are in the same phase where as in heterogeneous catalysis, the catalyst is in a different phase, mostly solid, than the reactants. In contrast to heterogeneous catalysis, homogeneous catalysis paves the way for deep understanding of their mechanisms and thus provides the opportunity for rational turning of the catalyst. Thus homogeneous or molecular catalysts offer improved selectivity, increased activity, and allow operationally lower temperatures.
1.2. Hydrogen and its Activation
Hydrogen molecule (H2) is the lightest element and is an abundant clean resource. The hydrogen atoms in H2 molecule is strongly held together by covalent bond and has dissociation energy of 104 kcal/mole. One of the common methods to activate this high strength non-polarized bond is by using transition-metal centres bearing suitable ligands. The
1 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
hydrogen molecule can coordinate to a metal centre in a side-on fashion (ɳ2) primarily via
donation of its two σ electrons to a vacant d orbital of the metal to form a stable dihydrogen complex. The first structurally characterized dihydrogen metal complex W(CO)3(Pi-Pr3)2(H2)
2 2 was discovered in 1983 by Kubas and co-workers. The stabilization of ɳ -H2 complexes arises from the back donation of electrons from a filled metal d orbital of metal to the σ* anti bonding orbital of H2. The back donation is analogous to that of Dewar-Chatt-Duncanson model for olefin coordination (Figure. 1.1).3
- - - C + - H
+ + M + - M +
+ H + C - + +
Figure 1.1. Dewar-Chatt-Duncanson model for olefin coordination (left) and bonding model for H2 coordination (right).
There are two modes of its activation; homolytic (considered as two H radicals) and
heterolytic splitting of dihydrogen (considered as a H- and a H+).
The homolytic splitting of dihydrogen leads to the oxidation of the metal leading to
the formation of metal dihydrides and thus oxidative addition of dihydrogen takes place. A
pictorial representation along with the H-H distance is depicted in Figure 1.2.
Figure. 1.2. Oxidative addition of H2 and H-H distances.
2 Chapter 1 General Introduction
Metal dihydrogen complexes have higher thermodynamic and kinetic acidity 2b,4 compared to most metal hydride complexes. H2 is a very weak acid (pKa = 35 in THF), 2 2 but ɳ -binding with a metal centre increases the acidity up to 40 times. The pKa of η -H2 can be as low as -6 and thus becoming acidic as strong as sulfuric acid or triflic acid.5 Therefore
Scheme 1.1. Heterolytic Splitting of H2.
the presence of a base can lead to the deprotonation from metal dihydrogen complex leading to “heterolytic” splitting of dihydrogen as depicted in Scheme 1.1.
1.3. Homogeneous Hydrogenation
Hydrogenation is one of the most extensively studied reactions in homogeneous catalysis.6
Homogeneous hydrogenation is often carried out with molecular hydrogen although much attention has been paid to derive hydrogen from other molecules which are capable of donating hydrogen, like alcohols, formic acid salts etc. The latter processes are termed as transfer hydrogenations.6 Transition metals are capable of exhibiting variable oxidation states which could be stabilized by a large variety of ligands. The ligands often appear are negative donors like hydrides, halides, alkyls or neutral donors like amines, imines, nitriles,
2 5 phosphines, carbon monoxide, ɳ -alkenes, ɳ -C5H5 or positive donors like nitrosyl etc. In this context, highly efficient and selective catalysts are available for the hydrogenation of unsaturated substrates including their asymmetric versions. Hydrogenation or related reduction reactions are often carried out with suitable orgaonsilanes which are termed as hydrosilylations. The developments in the area of organometallic chemistry paved the way for the preparation of a variety of metal complexes, particularly those of transition metals, active for hydrogenation and transfer hydrogenation as well as hydrosilylation reactions under mild conditions.
3 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
1.4. Hydrogenation of Various Functional Groups
1.4.1. Hydrogenation of Alkenes
The first documented example of homogeneous hydrogenation by metal compounds was
reported by Calvin in 1938, reporting that quinoline solutions of copper acetate, at 100 °C,
were found to be active catalysts for the hydrogenation of quinines7. The most significant
advances in homogeneous hydrogenation catalysis have been the discovery of rhodium
8 phosphine complex [RhH(CO)(PPh3)3] by Bath and Vaska in 1963. Later in few years, the catalytic activity of this complex for hydrogenation, isomerisation and hydroformylation reactions were reported by Wilkinson and co-workers.9 The most important rhodium catalyst, the [RhCl(PPh3)3] complex, was reported during the period 1965-1966 independently by
Wilkinson, Bennett and Vaska.10 Wilkinson and co-workers extensively studied the remarkable catalytic properties of this complex, which is usually known as Wilkinson’s catalyst. This turned out to be the first practical hydrogenation system working usually at room temperature and atmospheric pressure of hydrogen. During this time [IrCl(CO)(PPh3)2] was discovered by Vaska, called Vaska’s complex,11 which was susceptible for oxidative addition-reductive elimination with dihydrogen to form [IrH2Cl(CO)(PPh3)2] whose activity was very weak. Also, the iridium analogue of Wilkinson catalyst, [IrCl(PPh3)3] was also weakly active.10b The almost inactivity of these complexes towards hydrogenation were due
to inability to form vacant sites by dissociation of PPh3 ligand from [IrH2(PPh3)3] . For the
two decades, rhodium chemistry dominated in the field of hydrogenation, due to the
remarkable investigations of Wilkinson, Kagan, Osborn, and Knowles.12 Ruthenium was
slowly developing during these period starting with studies by Halpern6,13 and Wilkinson.6,14
In 1965, Wilkinson and co-workers found that the reaction of RuCl2(PPh3)3 with hydrogen
and a base gave the hydride complex RuHCl(PPh3)3, a very active catalyst for
hydrogenation.13 This monohydride complex is formed by the abstraction of proton from the
4 Chapter 1 General Introduction
acidic ɳ2-dihydrogen ruthenium complex by the base. This interpretation was arisen after the isolation of metal dihydrogen complexes by Kubas.2a However, this ruthenium system
4 operates at 0.66 bar of H2 pressure at 25 °C showing TOF of 10 in the hydrogenation of 1- octene. It is almost 20 times active when compared to the well-known Wilkinson’s catalyst
15 RhCl(PPh3)3 under similar conditions.
Halpern came up with a more promising mechanism for alkene hydrogenation16, and is supported by careful kinetic and spectroscopic studies of hydrogenation of cyclohexene.
The predominant hydride route consists of oxidative addition of a hydrogen molecule prior to alkene coordination (H2 before olefin; known as Wilkinson-type hydrogenation). The complex [RhCl(PPh3)3] undergoes dissociation of PPh3 to form the 14-electron species
RhCl(PPh3)2 (Scheme 1.2). The rapid oxidative addition of hydrogen to RhCl(PPh3)2,
Ph3P PPh3 Rh Ph3P Cl
-PPh3
Ph3P Rh Cl R H2 Ph3P
H Ph3P Rh H Ph3P Ph3P Cl R Rh H Ph3P Cl
H Ph3P H Rh H R Ph3P Ph3P Cl Rh H Ph P 3 Cl
R
Scheme 1.2: Wilkinson-type (H2 before olefin) mechanism of hydrogenation of olefins by RhCl(PPh3)3.
5 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
followed by alkene coordination, affords the 18-electron octahedral dihydride alkene
complex [RhH2Cl(alkene)(PPh3)2]. The rate-determining step for the whole process is the intramolecular alkene insertion into the rhodium-hydride bond of [RhH2Cl(alkene)(PPh3)2], to produce the alkyl hydride intermediate, [RhH(alkyl)Cl(PPh3)2]. The next step, the
reductive elimination of alkane from this alkyl hydride intermediate to regenerate occurs
rapidly. The proposed cycle implies changes in the oxidation state (I and III) in the oxidative
addition and reductive elimination.
In the case of saturated 18-electron complex [RhH(CO)(PPh3)3], dissociation of PPh3 and
creation of a vacant coordination site gives rise to the catalytic active species,
17 [RhH(CO)(PPh3)2] (Scheme 1.3). Coordination of the alkene substrate to
[RhH(CO)(PPh3)2] (Olefin before H2; known as Osborn-type hydrogenation) followed by the
Scheme 1.3: Osborn-type (olefin before H2) mechanism of hydrogenation of olefins by RhH(CO)(PPh3)2.
6 Chapter 1 General Introduction
insertion of the hydride to the coordinated alkene takes place to form the Rh(alkyl) species.
Oxidative addition of H2 followed by reductive elimination would regenerate the active species RhH(CO)(PPh3)2.
A large variety of homogeneous catalytic systems that efficiently hydrogenate a variety of olefins has been reported. Among them, the most active systems effective for 1- hexene hydrogenations are depicted in Table 1.2.
Table 1.2. Catalytic activity of several well-known Wilkinson and Osborn type catalysts in 1-hexene hydrogenation.18
Temp TOF Entry Catalyst Solvent H2 (bar) (°C) (h-1)
10b 1 C6H6/EtOH 25 1 650
Wilkinson catalyst
15 2 C6H6 25 1 9000
319,20 CH2Cl2 25 1 4000
Schrock-Osborn catalyst
20 4 CH2Cl2 0 1 6400
Crabtree catalyst
Solvent free 23 1 1725
518a Solvent free 23 10 17000
CH2Cl2 90 10 56000 Berke catalyst
7 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
618b Solvent free 23 10 6.0×105
Berke catalyst
The most significant step further was the use of chiral phosphines with Rh precursors, realizing catalytic enantioselective hydrogenation reported in 1968 by the groups of
Knowles21 and Horner.22
H Ph O PPh2 P PPh o- 2 PPh Anisyl O 2 PPh2 H PAMP (-)-DIOP (R)-BINAP Figure 1.3. Chiral ligands with different modes of origin of chirality used in asymmetric hydrogenation reactions.
A large number of subsequent publications have reported the development of catalytic systems containing a variety of chiral ligands for hydrogenation of a wide range of prochiral substrates including alkenes, ketones and ketimines. Processes reaching enantiomeric excess
Scheme 1.4. Examples of asymmetric hydrogenation of C=C bonds operating in industry.
8 Chapter 1 General Introduction
(ee) close to 100% are now a days common. Also, at least in few cases, their mechanisms
could be understood in detail. More than a dozen of industrial, catalytic enantioselective
homogeneous hydrogenation processes are now operating in fine and intermediate chemical
industries particularly for pharmaceuticals and agrochemicals. Few of such systems involving
C=C bond hydrogenation are given in Scheme 1.4.23
1.4.2. Hydrogenation of Aldehydes and Ketones
The reduction of carbonyl compounds to their corresponding alcohols is one of the most
fundamental and widely employed reactions in synthetic organic chemistry. Though
aluminium and boron hydride reagents are often used in laboratory for their reductions, in an
industrial and practical point of view, procedures that make use of molecular hydrogen show
better ecology, are more cost-effective, and are potentially easier to operate than those require
the clean-up of boron or aluminium waste at the end of the reaction.
1.4.2.1. Aldehydes
The first report of a catalytic system for the effective homogeneous hydrogenation of an
aldehyde to an alcohol was reported in 1967.24 Coffey reported that the use of a catalyst prepared in situ by the reaction of [Ir(H)3(PPh3)3] with acetic acid was effective for the hydrogenation of n-butyraldehyde to n-butanol at 50 °C and under 1 bar of H2 pressure. This
catalytic system was further studied by Strohmeier and Steigerwald, who performed reactions
at 10 bar without solvent to achieve hydrogenation of a series of aldehydes.25 Turnover numbers (TON) of up to 8000 were achieved in the case of the hydrogenation of benzaldehyde. Wilkinson catalyst [RhCl(PPh3)3], a convenient catalyst for the hydrogenation
of olefins, was found to be deactivated by aldehydes to give the catalytically inactive
10b complex [RhCl(CO)(PPh3)2] as a result of the competing decarbonylation reaction.
Strohmeier and Weigelt used the catalyst [RuCl2(CO)2(PPh3)2] to hydrogenate a series of
9 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
26 aldehydes at 15 bar H2 and at 160-180 °C, with generally high yield and turnover numbers;
TONs of upto 56000 were achieved in the hydrogenation of benzaldehyde and that of 59400 were achieved in the hydrogenation of 2-methylpentanal. Although these are amongst the highest turnover numbers reported for aldehyde hydrogenation, the reactions were carried out at relatively high temperatures. [RuHCl(CO)(PPh3)3] when used in the hydrogenation of propionaldehyde with a SCR of 50 000, TONs of up to 32000 were achieved after 50 h at 140
27 °C under 30 bar H2. Using this same catalyst in the reduction of crotonaldehyde, the favoured product was the fully saturated alcohol.28
1.4.2.2. Ketones
+ [Rh(bpy)2] , obtained by the in situ reduction of [Rh(bpy)2Cl2]Cl with hydrogen in
29 methanolic sodium hydroxide, can reduce a series of simple ketones under 1 bar H2 and at
30 30 °C. One of the notable catalytic system Ru(Cl)2((S)-tolbinap)((S,S)-dpen)/t-BuOK syatem designed by Noyori and co-workers could hydrogenate acetophenone in TON of 2.4 x
6 5 10 with TOF of 2x 10 under a H2 pressure of 45 bar at 30 °C, providing 80% ee in 2-
31 propanol as solvent. The complex [RuCl2(PPh3)3] was also active for the hydrogenation of ketones and Noyori and coworkers in 1995 found out that the activity of this complex could be enhanced by the addition of ethylenediamine (en) and KOH/i-PrOH.32 Using this system, with a catalyst loading of 0.02 mol%, Ru: en: KOH, 1 :1 : 20, at 28 °C under 3 bar H2, TOFs of 6700 h-1 were realized in the hydrogenation of acetophenone. By increasing the pressure to
50 bar and using a SCR of 10 000, TOFs in excess of 23 000 were obtained. This system was even shown to work at -20 °C, indicating the mildness of the conditions. The screening of various amines revealed that at least one primary or secondary amine end was necessary along with a base to improve activities. Subsequently Noyori and coworkers isolated the stable precatalysts amino phosphine complexes trans-[RuCl2(phosphane)2(1,2-diamine)]
10 Chapter 1 General Introduction
Scheme 1.5. Noyori’s metal-ligand bifunctional catalysis for hydrogenation of ketones applying a
RuCl2(PR3)2(diamine)/base system in 2-propanol. (X = H, or alkoxy/amino). which showed more rapid hydrogenation of ketones than the in situ systems.33 These findings
paved a break-through in the field with the concept of ‘metal-ligand bifunctional catalysis’
with outer sphere coordination mechanism (Scheme 1.5).
The years after witnessed the application of this concept emerging a large number of
documentations particularly in the area of enantioselective hydrogenations and transfer
hydrogenations of ketones and ketimies. One of the enantioselective ketone hydrogenations
operating in industry is shown in Scheme 1.6.23a,e
Scheme 1.6. Example of asymmetric hydrogenation of C=O operating in industry.
However, the classical mechanism would operate in metal complexes bearing at least
a hydride ligand and a vacant site cis to each other. Linn and Halpern proposed a mechanism
34 involving such a type of species. They found that H2 dissociates from RuH4(PPh3)3 (later
2 found as (Ru(ɳ -H2)H2(PPh3)3) when the ketone coordinates. Coordination of the ketone to
the ruthenium hydride species followed by the insertion of it into the Ru-H bond would
generate an alkoxide intermediate RuH(OR)(PPh3)3 (Scheme 1.7). Hydrogen coordination
11 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
H XH X [Ru] R' R'' R' R'' -L +L
H R' H H R' HH R'' [Ru] L [Ru] X R'' [Ru] X
R' H R'' [Ru] X H2
Scheme 1.7: Classical mechanism of hydrogenation of aldehydes, ketones and imines. [Ru]=RuH(PPh3)3, L=H2; X = O, NR). followed by the rapid elimination of alcohol would regenerate the ruthenium hydride species.
The same mechanism is operative also for aldehydes and imines.
1.4.3. Reductive Amination and Imine Hydrogenation
Aldehydes or ketones react with amines to form carbinolamines or imines which are subsequently reduced to substituted amines are called reductive amination with respect to carbonyl compound or reductive alkylation with respect to the amine. This reaction using sodium borohydride or sodium cyanoborohydride is well established. However, a more environmentally benign, economical and practical method to carry out this reaction is to use molecular hydrogen. Though several heterogeneous catalysts have been shown to be effective in this transformation, the main focus is to use more controllable homogeneous catalysts. The first example of this type of transformation was reported by Mark′o and Bakos in 1974 using
Co and Rh carbonyls35. In 2000, Borner and coworkers described a more practical catalytic system for these reactions.36 Reductive alkylation of piperidine with benzaldehyde could be achieved using [Rh(dppb)(COD)]BF4 or [Rh(1,2-bis-diphenylphosphinitoethane)(COD)]BF4 under mild conditions 50 bar H2 at room temperature with moderate selectivity towards the
12 Chapter 1 General Introduction
desired tertiary amine with the formation of the corresponding alcohol. Beller and co-workers reported a more practical system for reductive amination of aromatic aldehydes using
37 ammonia. [Rh(cod)Cl]2 along with TPPS ligand and NH4OAc furnished 86% of
-1 benzylamine with a TOF of 1720 h under 50 bar H2 at 135 °C.
The reaction of RuCl2(PPh3)3 with diphosphines with medium bite angles (dppb, diop, binap) produces complexes RuCl2(diphosphine)(PPh3) that are used as catalysts for the hydrogenation of imines.38 The dppb complex can be converted to the binuclear dihydrogen
2 complex (ɳ -H2)(dppb)Ru(µ-Cl)3Ru(dppb)Cl, which is a precatalyst for the hydrogenation of
39 aldimines. The PPh3 ligands in RuHCl(PPh3)3 can be displaced with P-N ligands produce a range of analogous precatalysts such as RuHCl(diamine)(PPh3)2 and trans-
RuHCl(diamine)(diphosphine). When the former diamine compound is activated with
40 alkoxide base under H2, it is an active catalyst for ketone and imine hydrogenation, while the latter is a precatalyst for the asymmetric hydrogenation of imines and ketones under mild conditions 41 both operating through ‘metal-ligand bifunctional catalysis.
At this point, it worth mentioning the synthesis of the herbicide S-Metalachlor by the
P(3,5-xylyl)2
Fe PPh2 Xyliphos
Scheme 1.8. Synthesis of S-Metalachlor by the asymmetric hydrogenation of a C=N bond.
13 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
enantioselective hydrogenation of an imine using an [Ir(COD)Cl]2/Xyliphos system
6 developed by Blaser and co-workers, resulting in an extremely high TON = 2×10 , TOF =
5 -1 4×10 h (Scheme 1.8).42 However, the enantioselectivity does not exceed 80%. For an
agrochemical, this enantioselectivity proved to be sufficient. This process is used industrially,
presently run on a scale of 10,000 ton/year and is the largest asymmetric catalytic process
today. It is the first example of asymmetric imine hydrogenation that was successfully
applied in industry.
1.4.4. Hydrogenation of Nitriles
Nitriles are one of the most difficult classes of substrates to hydrogenate.43 When they are subjected to hydrogenation, concomitant with the reduction processes, crucial selectivity probles arises: the formation of mixtures of primary, secondary and tertiary amine, as well as intermediate imines.44 Homogeneous hydrogenation of nitriles to primary amines were
reported by Beller and co-workers applying in situ Ru(COD)(methylallyl)2/DPPF catalytic
45 system at 50 bat H2 at temperatures of 80-140 °C. At 140 °C, TOF of up to 4800 h-1 were
realized with yields close to 100%. A notable example of hydrogenation of nitriles to primary
2 amines was documented by Sabo-Etienne and co-workers using Ru(ɳ -H2)2H2(PCy3)2. Under
mild conditions of 3 bar of H2 at 22 °C, 0.5 mol% of the catalyst provided 98% yield of the
primary amine in 2 h (TOF: 98 h-1).46 Quite recently, Milstein and co-workers reported a
Ru(PNN) system operating at 4 bar H2 pressure at 70 °C furnishing secondary imines with
good selectivities.47
1.4.5. Hydrogenation of Carboxylic esters
The hydrogenation of acids and esters using molecular hydrogen is generally a difficult task.
Lithium aluminum hydride and certain boron hydrides are traditionally used for this
reduction. However, the use of a stoichiometric aluminium reagent is not atom-economical
14 Chapter 1 General Introduction
and requires the separation and disposal of large quantity of waste. As an ideal and green
alternative to any of the stoichiometric procedures, catalytic hydrogenation using molecular
hydrogen is a versatile tool and would attract industrial attention if a catalyst were
sufficiently active. Heterogeneous catalysts capable of carrying out this process operate under
very harsh conditions which limits their application. Grey et al reported the first
homogeneous hydrogenation of an ester to alcohol.48 The complex
K2[Ru2(PPh3)3(PPh2)H4]2.diglyme could hydrogenate methyltrifluoroacetate to trifluoroethanol and methanol at 90 °C under 6 bar H2. However, formate esters were found to decompose with the liberation of carbon monoxide under these reaction conditions.
Milstein and co-workers reported the hydrogenation of methyl formate, dimethyl carbonate and carbamates using a Ru(PNN)(CO) system. Methyl formate could be obtained in TON of 4700 with 97% yield when a pressure of 50 bar at 110 °C was adopted.49 The mechanism proceeds through H2 splitting resulting in the formation of Ru-H and a C-H where it is driven by the aromaticity of an attached tridentate pyridyl ligand. This is popularly known as Milstein’s aromatization-dearomatization principle (Scheme 1.9).
H H
H PR' +H H PR' N 2 2 N 2 Ru Ru -H2 CO X CO X H R R2 2 X = N, P; R = i-Pr, t-Bu
Scheme 1.9. Activation of H-H bond by Milstein's PNN and PNP based Ru system.
Ester hydrogenation catalyzed by a ruthenium(II) complex bearing an N-heterocyclic carbene tethered with an NH2 operating through metal-ligand bifunctional catalysis has been
15 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
50 -1 reported by Morris et al recently. TOFs of up to 1500 h were accomplished at a H2 pressure of 25 bar at 50 °C in the hydrogenation of phthalide.
1.4.6. Hydrogenation of Carbon Dioxide and Carbonates/Bicarbonates
The inexpensive, abundant, low toxic CO2 gas is one of the major reasons for green house effect and thereby leading to global warming and climatic changes.51 The burning of fossil fuels to serve the energy demands of the world has been led a greater extend to the accumulation of this gas.52 The future energy demand rely on long lasting or renewable methods since it is estimated that the fossil fuel sources will deplete in the near future.53 In these contexts, production of synthetic fuel from climate threatening sources would be a method of choice and as a step; the hydrogenation of carbon dioxide to methanol, a C1 feed stock, is highly demanding to run the daily needs of the future, there by tackling the issue of
54 global CO2 emissions.
Hydrogenation of CO2 to methanol has been reported with heterogeneous catalytic systems. Most prominent among them are the Cu-Zn based systems that operate at high temperatures and pressures.55 In recent past, tremendous efforts are being made in academia to develop catalytic systems that are capable of reducing CO2 to formates using well defined
Ru, Rh and Ir systems.56 Notable catalyst among these is an Ir-PNP pincer ligand system, reported recently by Nozaki and coworkers, for which TON and TOFs up to 3.5 x 105 and 1.5
5 57 x 10 respectively, were achieved. Reduction of CO2 to methanol has been achieved using boranes, phosphaboranes and silanes, but apart from the issue of high cost, these reagents lead to the formation of large amount of waste.58 Preliminary outcome of efforts on metal catalyzed homogeneous reduction of CO2 to MeOH was reported recently by Huff and
Sanford through ruthenium catalyzed cascade reaction involving formic acid and methyl formate as intermediates.59 In this report, different ruthenium complexes reported to be
16 Chapter 1 General Introduction
capable of catalyzing CO2 to formate level, and formate esters to methanol were rationally sequenced along with an acid co-catalyst, the latter was added to enhance esterification, all carried out in a single reaction vessel to effect this transformation. Quite recently, Leitner and co-workers demonstrated this reaction using ruthenium-phosphine catalytic system, which was already found to be efficient for the hydrogenation of carboxylic acids and their derivatives to the corresponding alcohols.60 They reported a maximum TON of 221 under
CO2 pressure of 20 bar and H2 pressure of 60 bar at 140 °C using a RuP3/acid system. The role of acid was to enhance the esterification of the intermediate formic acid.
Recent years witnessed an emergence in the hydrogenation of bicarbonates and carbonates to formate salts. Like CO2, the reductions of bicarbonates are also of considerable interest, because CO2 can be easily trapped from waste streams in basic solution. Beller and co-workers recently developed a reversible energy storage system in which bicarbonates and carbonates is hydrogenated to the corresponding formates and the latter releases hydrogen on
61 demand to form again bicarbonate. A [{RuCl2(benzene)}2] and dppm system at a H2 pressure of 80 bar furnished sodium formate in TON of 1100 and TOFs of 550 h-1 at a
temperatures of 70 °C. The H2 release was carried out at 40 °C with the same catalyst furnishing it in TON of 2000 with initial TOF’s up to 2900 h-1. In 2012, the same group
reported a tetradentate FeP4 system capable of hydrogenating NaHCO3 to NaHCO2 with TON
-1 62 of 7500 and TOF of 750 h under a H2 pressure of 60 bar at 100 °C.
1.5. Transfer Hydrogenation
The transfer hydrogenation particularly the asymmetric transfer hydrogenation of ketones and
imines is a useful tool in organic synthesis. Like catalytic hydrogenation, catalytic transfer
hydrogenation is also considered to be an environmentally friendly transformation, the latter
even much safer and convenient to handle on any scale. The first homogeneous transfer
17 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
hydrogenation was reported in 1925 when Meerwein and Schmidt described the reduction of ketones and aldehydes using alcohols as reductants and aluminum alkoxides as the catalysts.63 The major difference from previous studies was the hydrogen source; instead of molecular hydrogen, a small organic molecule was utilized to provide the hydrogen necessary to reduce the carbonyl compound. A decade later, Oppenauer recognized the possibility of reversing the reaction into an oxidation procedure.64
1.5.1. Transfer hydrogenation of Ketones and Imines
Although actually designed for hydrogenation with molecular hydrogen, Wilkinson catalyst
(RhCl(PPh3)3) has also been used in transfer hydrogenation catalysis using 2-propanol as hydrogen donor.10b Today, several transition metal catalysts are available to perform the transfer hydrogenation of ketones and imines with acceptable efficiency particularly using 2- propanol as hydrogen donor. Though the classical primary coordination sphere mechanisms operating through the availability of a metal hydride and a vacant site are extensively reported on ruthenium monodentate phosphine systems65 there has been a large number of literature on transition metal catalyzed transfer hydrogenation reactions designed to operate through Noyori66 or Shvo67 type secondary coordination sphere metal ligand bifunctional mechanisms with simultaneous proton and hydride transfers.
General mechanism for the classical type of transfer hydrogenations of ketones and imines is depicted in Scheme 1.10.68
Pàmies and Bäckvall65k,n have proposed an alternative mechanism for transfer hydrogenations catalyzed by the dihydride RuH2(PPh3)3 that is thought to be formed by the reaction of the precatalyst RuCl2(PPh3)3 with base and 2-propanol (Scheme 1.11).
The key difference in these mechanisms is the generation of the reduced product, alcohol, through the protonation of the metal alkoxide species by 2-propanol or the reductive
18 Chapter 1 General Introduction
Scheme 1.10. Conventional mechanism of transfer hydrogenation of aldehydes, ketones and imines.
elimination of the alcohol from the metal(alkoxide) hydride species the former does not change the oxidation change of the metal throughout the catalytic cycle where as the latter leading to an oxidation change of the metal.
Scheme 1.11. Mechanism of transfer hydrogenation of aldehydes, ketones and imines catalyzed by
RuCl2(PPh3)3.
19 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Numerous reports on the transfer hydrogenation of ketones and imines operating
through the classical inner coordination sphere transfer hydrogenation mechanism as well as
outer coordination sphere metal ligand bifunctional mechanism are documented in
literature.6,69 The key interest in the recent research in this area of both ketone and imine
transfer hydrogenation is in developing their highly efficient asymmetric versions.
1.5.2. Transfer Hydrogenation of Nitriles
Like catalytic hydrogen hydrogenation of nitriles, the catalytic transfer hydrogenation of nitriles is also a challenging aspect and only a very few number of literature is available in this area. The complex RuH2(PPh3)4 catalyzes the transfer hydrogenation of benzonitrile to a mixture of the amines and imines; benzylamine (6%), the imine N-benzylidenebenzylamine
(20%) and the secondary amine, dibenzylamine (25%) in 51% conversion when a loading of
1.25 mol% at 85 °C run for 80 h.65e Recently, Beller and co-workers developed a promising method for the transfer hydrogenation of various aromatic as well as secondary and tertiary nitriles in the presence of [{Ru(pcymene)Cl2}2]/DPPB /Base system in 2-butanol at 120 °C.
TON of 280 h-1 could be achieved with moderate to good yields in most cases.70 Quite recently, the same group reported the transfer hydrogenation of nitriles followed by subsequent N-monoalkylation to secondary amines.71
The RuCl2(PPh3)3/NaOH/2-propanol(large excess) catalytic system at a temperature of 120
°C could furnish the N-isopropyl secondary amines in moderate to good yields.
1.6. Hydrosilylation of Nitriles
Transition metal catalyzed homogeneous hydrosilylations of aldehydes and ketones as well as imines are well established.72 Unlike Si-H addition to C=X bond (X = C, O, NR), the CN triple bond remains a great synthetic challenge.73 Hydrosilylation of nitriles also presents a difficult chemoselectivity problem in that the products of monoaddition, N-silylaldimines
20 Chapter 1 General Introduction
R3Si-N=CHR, are generally much more reactive than nitriles, so that the reaction proceeds
74 75 further to give the disilylamines (Rʹ3Si)2NCH2R. Since the report by Calas et al on the
ZnCl2-catalyzed condensation of HSiEt3 with PhCN to give the imine PhHC=NSiEt3 in moderate yield of 54 at 140-150 °C, only a few catalytic monohydrosilylations of nitriles have been published.74a,76 A notable one among them is the complex [Cp(i-
77 Pr3P)Ru(NCCH3)2]BAF (BAF=[B(C6F5)4]) reported by Nikonov and co-workers. Simple
alkyl and aryl nitriles were easily converted at room temperature into the corresponding N-
silylimines. Excellent chemoselectivity could be observed in the presence of C=C and C=O
functional groups. TOF’s of up to 75 h-1 were realized in this transformation.
1.7. Claisen-Tishchenko Reaction of Aldehydes
Ester synthesis and hydrogenation of organic substrates are among the many fundamental
transformations of fine chemical industry. The atom economic Claisen-Tishchenko
disproportionation of aldehydes to the corresponding carboxylic esters78 has acquired wide
attention due to their application in food, polymer, dye and perfume industry.79 Traditional
catalysts for this reaction include mainly sodium80 and particularly aluminium alkoxides.81
Various transition metal and lanthanoid compounds are reported to be active for Claisen-
Tishchenko reaction, one of the notable candidate among them is the transition metal RhIII
hydrido complex [Rh(PhBP3)(H)2(CH3CN)], reported by Tejel and co-workers, which with
applying 1 mol% at room temperature furnished the corresponding esters upto quantitative
yield in 1 min. (TOF: 6000 h-1).82 Recently, this disproportionation reaction between two
different selected aldehydes could be accomplished in good selectivities using a metal
complex of Ni.83 The key step involved in Claisen-Tishchenko reaction is the hydride transfer
from one molecule of aldehyde to another molecule of aldehyde and subsequent coupling
between the two species. Thus, one can expect that metal complexes capable of performing
21 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
the transfer hydrogenation reactions can often also catalyze the disproportionation reactions
of aldehydes to the corresponding carboxylic esters.
1.8. Goal of the Project
The area of homogeneous hydrogenations is dominated by platinum group metals.6a Due to their toxicity, the precious metal components after the catalysis are to be removed from the active pharmaceutical ingredients in industrial processes.84 Also, due to their scarcity and high cost, these catalysts need to be recycled. Precious metal catalysis often suffers from reduction to the metals under hydrogenation conditions resulting in loss of catalytic activity.85
Being border to the precious metals in the periodic table, the element rhenium is expected to
86 show some of the precious metal’s character. This is revealed in their interaction with H2
and olefins.87 The most active catalytic homogeneous hydrogenation and related reactions
consists of generally ruthenium phosphine or carbonyl or rhodium halide fragments.6 A
rhenium nitrosyl fragment is isoelectronic with them. These properties made us believe that
suitable rhenium complexes could be efficient catalysts for hydrogenation and other related
reactions.
Nitrosyl ligand can supports different oxidation states of metal centers often accompanied
by different coordination modes.19,88 Furthermore it exerts a relatively strong trans-effect,
leading to activation of metal−ligand bonds. One of the latter influences is nitrosyl-
substituted transition metal hydrides in which the M−H bonds show increased hydridic
character.19,88 Halogen ligands like a bromide are good π-donors and disposing them trans to
the strong π-acceptor NO ligand would exert a stabilizing strong push pull π-interaction
which would leads to the other cis ligands labile.89
Diphosphines are often applied as ligands with precious metals active for hydrogenation
and related reactions.90 The PMP angle in a metal diphosphine complex termed as ‘bite
22 Chapter 1 General Introduction
P NO H M
P Br or Rhenium Nitrosyls are Isoelctronic with Ruthenium Carbonyls/Phosphines or Rhodium Halides
Scheme 1.12. Influence of bite angle on steric and electronic factors.
angle’ can have an influence on the steric and electronic factors - a ligand tuning would help angle’ can have an influence on the steric and electronic factors - a ligand tuning would help to increase the efficiency of the catalyst.91 A wide bite angle of the ligands induces a distortion from the octahedral geometry that can lead to poor orbital overlap between the metal and the bound atoms (Scheme 1.12). A small bite angle ligand, though generally not hindered, can also lead to poor orbital overlapping of the bound atoms attached with metal centre due to the distortion.
The Xantphos family of ligands are often used in many catalytic transformations, particularly in hydroformilation reactions.91,92 Also, silicon containing compounds are usually found to have high stability. 4,6-Bis(diphenylphosphino)-10,10-dimethylphenoxasilin
(Sixantphos)92 a diphosphine ligand belonging to the xantphos family of ligands bearing a silicon back bone having a large bite angle of 108° would be a suitable choice for the preparation of the catalyst. Complexes bearing other large bite angle diphosphines; the new derivative of Sixantphos ligand 4,6-Bis(diphenylphosphino)-10,10-diphenylphenoxasilin
23 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
(Sixantphos-Ph2) as well as one with a sulfur backbone 4,6- bis(diphenylphosphino)phenoxathiin (Thixantphos) were also prepared.
The classical well known Wilkinson or Osborn type catalysis consists of a metal hydride and a ɳ2-olefin/vacant site coordination during the catalytic cycle. The actual hydrogenation of these catalytic cycles starts at this state by the insertion of olefin into the M-
H bond. Thus, preparation of the catalytic active species or a species much closer to the active species would certainly be one of the best choices to start with. Keeping these in mind, we have targeted the synthesis of appropriate nitrosyl rhenium complexes bearing large bite angle Sixantphos ligand with ethylene and a H cis to each other and both trans to the diphosphine and a bromide in the other position trans to the NO ligand (Scheme 1.13).
Scheme 1.13. Type of target Re(I) complexes for catalytic hydrogenation and related reaction.
1.9. References
1. “Recognizing the Best in Innovation: Breakthrough Catalyst”. R&D Magazine, Sep. 2005, p. 20. 2. a) G. J. Kubas , R. R. Ryan , B. I. Swanson , P. J. Vergamini, H. J. Wasserman J. Am. Chem. Soc. 1984, 106, 451-452; b) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes: Structure, Theory, and Reactivity, Kluwer, New York, 2001; (c) Kubas, G. J. Chem. Rev. 2007, 107, 4152. 3. (a) M. J. S. Dewar, Bull. Soc. Chim. Fr. 1951, 18, C79; b) J. Chatt, L. A. Duncanson, J. Chem. Soc. 1953, 2929; c) R. E. Harmon, S. K. Gupta, D. J. Brown, Chem. Rev. 1973, 73, 21-52; d) Kubas, G. J. J. Organometal. Chem. 2001, 635, 37. 4. a) P. G. Jessop, R. H. Morris, Coord. Chem. Rev. 1992, 121, 155-284. b) D. M. Heinekey, W. J. Jr. Oldham, Chem. Rev. 1993, 93, 913-926. 5. a) R. H. Morris, Can. J. Chem. 1996, 74, 1907. b) G. Jia, C. P. Lau, Coord. Chem. Rev. 1999, 83, 190- 192. 6. a) J. G. de Vries, C. J. Elsevier, in Handbook of Homogeneous Hydrogenation; Eds.; Wiley-VCH: Weinheim, 2007; b) S. E. Clapham, A. Hadzovic, R. H. Morris, Coord. Chem. Rev. 2004, 248, 2201- 2237. 7. a) M. Calvin, Trans. Far. Soc. 1938, 34, 1181; b) M. Calvin, J. Am. Chem. Soc. 1939, 61, 2230.
24 Chapter 1 General Introduction
8. a) S. S. Bath, L. Vaska, J. Am. Chem. Soc. 1965, 85, 3500; b) L. Vaska, Inorg. Nucl. Chem. Lett. 1965, 1, 89. 9. a) D. Evans, G. Yagupsky, G. Wilkinson, J. Chem. Soc. A 1968, 2660; b) M. Yagupsky, C.K. Brown, G. Yagupsky, G. Wilkinson, J. Chem. Soc. A 1970, 937; c) C. O’Connor, G. Yagupsky, D. Evans, G. Wilkinson, Chem. Commun. 1968, 420; d) C. O’Connor, G. Wilkinson, J. Chem. Soc. A 1968, 2665; e) G. Yagupsky, G. Wilkinson, J. Chem. Soc. A 1970, 941; f) D. Evans, J. A. Osborn, G. Wilkinson, J. Chem. Soc. A 1968, 3133; g) G. Yagupsky, C. K. Brown, G. Wilkinson, Chem. Commun. 1969, 1244; h) G. Yagupsky, C. K. Brown, G. Wilkinson, J. Chem. Soc. A 1970, 1392; i) C. K. Brown, G. Wilkinson, J. Chem. Soc. A 1970, 2753. 10. a) J. A. Osborn, G. Wilkinson, J. F. Young, Chem. Commun. 1965, 17; b) J. A. Osborn, F. H. Jardine, J. F. Young, G. Wilkinson, J. Chem. Soc. A 1966, 1711; c) M. A. Bennett, P. A. Longstaff, Chem. Ind. (London) 1965, 846; d) R. S. Coffey, British Patent 1121642, 1965; e) L. Vaska, R. E. Rhodes, J. Am. Chem. Soc. 1965, 87, 4970. 11. L. Vaska, D. Rhodes, J. Am. Chem. Soc. 1965, 87, 4970. 12. J. M. Brown, P. A. Chaloner, In Homogeneous Catalysis with Metal Phosphine Complexes, Pignolet, L.H. (Ed.), Plenum Press, New York, 1983, Ch 4. 13. J. Halpern, J. F. Harrod, B. R. James, J. Am. Chem. Soc. 1961, 83, 753. 14. D. Evans, J. A. Osborn, F. H. Jardine, G. Wilkinson, Nature 1965, 208, 1203. 15. P. S. Hallman, B. R. McGarvey, G. Wilkinson, J. Chem. Soc. A 1968, 3143. 16. Joshi, A. M., Macfarlane, K. S., James, B. R., J. Organomet. Chem. 1995, 488, 161. 17. a) C. O’Connor, G. Yagupsky, D. Evans,G. Wilkinson, Chem. Commun. 1968, 420; b) C. O’Connor, G. Wilkinson, J. Chem. Soc. A 1968, 2665. 18. a) Y. Jiang, J. Hess, T. Fox, H. Berke, J. Am. Chem. Soc. 2010, 132, 18233-18247. b) Y. Jiang, B. Schirmer, O. Blacque, T. Fox, S. Grimme, H. Berke, J. Am. Chem. Soc. 2013, 135, 4088-4102; c) Crabtree, R. H. Acc. Chem. Res. 1979, 12, 331. 19. a) R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 2134; b) R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 4450; c) R. H. Crabtree, A. Gautier, G. Giordano, and T. Khan, J. Organometal. Chem. 1977, 141, 113. 20. R. H. Crabtree, H. Felkin, and G. E. Morris, J. Organometal. Chem. 1977, 141, 205. 21. W.S. Knowles, M. J. Sabacky, Chem. Commun. 1968, 1445. 22. a) L. Horner, H. Siegel, H. Büthe, Angew. Chem. 1968, 80, 1034; b) Angew. Chem., Int. Ed. Engl. 1968, 7, 942. 23. a) H.U. Blaser, F. Spindler, M. Studer, Appl. Catal. A: Gen. 2001, 119-143 b) W.S. Knowles, Chem. Ind. (Dekker), 1996, 68, 141; c) W.S. Knowles, Acc. Chem. Res. 1983, 16, 106. d) W.S. Knowles, J. Chem. Ed. 1986, 63, 222. e) R. Schmid, M. Scalone, in: E. N. Jacobsen, H. Yamamoto, A. Pfaltz (Eds.), Comprehensive Asymmetric Catal. Springer, Berlin, 1999, p. 1439. 24. R. S. Coffey, J. Chem. Soc. Chem. Commun. 1967, 923. 25. W. Strohmeier, H. Steigerwald, J. Organomet. Chem. 1977, 129, 43. 26. W. Strohmeier, L. Weigelt, J. Organomet. Chem. 1978, 145, 189.
25 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
27. R. A. Sanchez-Delgado, A. Andriollo, O.L. De Ochoa, T. Suarez, N. Valencia, J. Organomet. Chem. 1981, 209, 77. 28. R. A. Sanchez-Delgado, A. Andriollo, N. Valencia, J. Mol. Catal. 1984, 24, 217. 29. G. Mestroni, G. Zassinovich, A. Camus, J. Organomet. Chem. 1977, 140, 63. 30. G. Mestroni, R. Spogliarich, A. Camus, F. Martinelli, G. Zassinovich, J. Organomet. Chem. 1978, 157, 345. 31. R. Noyori, T. Ohkuma, Angew. Chem. Int. Ed. Engl. 2001, 40, 40. 32. T. Ohkuma, H. Ooka, S. Hashiguchi,T. Ikariya, R. Noyori, J. Am. Chem. Soc. 1995, 117, 2675. 33. (a) H. Doucet, T. Ohkuma, K. Murata, T. Yokozawa, M. Kozawa, E. Katayama, A. F. England, T. Ikariya, R. Noyori, Angew. Chem. 1998, 110, 1792-1796. 34. Linn, D.E., Halpern, J., J. Am. Chem. Soc. 1987, 109, 2969. 35. L. Mark′o, J. Bakos, J. Organomet. Chem. 1974, 81, 411. 36. V. I. Tararov, R. Kadyrov, T.H. Riermeier, A. Borner, J. Chem. Soc. Chem. Commun. 2000, 1867. 37. T. Gross, A.M. Seayad, M. Ahmad, M. Beller, Org. Lett. 2002, 4, 2055. 38. D. E. Fogg, B. R. James, In: Catalysis of Organic Reactions of the Chemical Industry. Dekker, 1995, 62, 435. 39. K. S. Macfarlane, I. S. Thorburn, P. W. Cyr, D. Chau, S. J. Rettig, B. R. James, Inorg. Chim. Acta 1998, 270, 130. 40. R, Abbel, K. Abdur-Rashid, M. Faatz, A. Hadzovic, A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2005, 127, 1870. 41. K. Abdur-Rashid, A. J. Lough, R. H. Morris, Organometallics 2001, 20, 1047. 42. H. U. Blaser, H. P. Buser, K. Coers, R. Hanreich, H. P. Jalett, E. Jelsch, B. Pugin, H. D. Schneider, F. Spindler, A. Wegmann, Chimia 1999, 53, 275. 43. a) R. A. Grey, G. P. Pez, A. Wallo, J. Am. Chem. Soc. 1981, 103, 7536 – 7542; b) T. Yoshida, T. Okano, S. Otsuka, J. Chem. Soc., Chem. Commun. 1979, 870 – 871. 44. a) S. Gomez, J. A. Peters, T. Maschmeyer, Adv. Synth. Catal. 2002, 344, 1037-1057; b) B. Chen, U. Dingerdissen, J. G. E. Krauter, H. G. J. L. Rotgerink, K . Mobus, D. J. Ostgard, P. Panster, T. H. Riermeier, S. Seebald, T. Tacke, H. Trauthwein, Appl. Catal. A: Gen. 2005, 280, 17-46; c) R. Reguillo, M. Grellier, N. Vautravers, L. Vendier, S. Sabo-Etienne, J. Am. Chem. Soc. 2010, 132, 7854-7855; d) J. von Braun, G. Blessing, F. Zobel, Ber. Dtsch. Chem. Ges. 1923, 56, 1988-2001; e) G. Mignonac, Comptes Rendus 1920, 171, 14. 45. a) S. Enthaler, D. Addis, K. Junge, G. Erre, M. Beller, Chem. Eur. J. 2008, 14, 9491-9494. 46. R. Reguillo, M. Grellier, N. Vautravers, L. Vendier, and S. Sabo-Etienne, J. Am. Chem. Soc. 2010, 132, 7854–7855 47. D. Srimani, M. Feller, Y. Ben-David, D. Milstein, Chem. Commun. 2012, 48, 11853-11855. 48. R. A. Grey, G.P. Pez, A. Wallo, J. Am. Chem. Soc. 1981, 103, 7536. 49. E. Balaraman, C. Gunanathan, J. Zhang1, L. J. W. Shimon, D. Milstein, Nat. Chem. 2011, 3, 609-614. 50. W. N. O. Wylie, R. H. Morris, ACS Catal. 2013, 3, 32-40. 51. IPCC Fourth Assessment Report: Climate Change 2007: Synthesis Report; Ch. 2.2.
26 Chapter 1 General Introduction
52. Annual Energy Review 2011; U.S. Energy Information Administration: Washington, DC, 2012; Table 11.1, pp 302-303. 53. USGS World Petroleum Assessment 2000 and US DOE IEA 1999, World Energy Overview. 54. N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. 2006, 103, 15729. 55. a) K. Ushikoshi, K. Mori, T. Watanabe, M. Takeuchi, M. Saito, Stud. Surf. Sci. Catal. 1998, 114, 357; b) M. Saito, Catal. Surv. Jpn. 1998, 175. c) L. C. Grabow, M. Mavrikakis, ACS Catal. 2011, 1, 365. 56. a) Iyad Karamé. Hydrogenation; Ch 10: InTech: Rijeka, Croatia, 2012. (b) P. G. Jessop, F. Joó, F, C. - C. Tai, Coord. Chem. Rev. 2004, 248, 2425-2442. (c) W. Wang, S. Wang, X. Ma, J. Gong, Chem. Soc. Rev. 2011, 40, 3703-3727. 57. R. Tanaka, M. Yamashita, K. Nozaki, J. Am. Chem. Soc. 2009, 131, 14168-14169. 58. a) S. Chakraborty, J. Zhang, J. A. Krause, H. Guan, H. J. Am. Chem. Soc. 2010, 132, 8872; b) S. N. Riduan, Y. Zhang, J. Y. Ying, Angew. Chem. 2009, 121, 3372-3375; c) M.-A. Courtemanche, M.-A. Légaré, L. Maron, F.-G. Fontaine, J. Am. Chem. Soc. 2013, 135, 9326-9329. 59. C. A. Huff, M. S. Sanford, J. Am. Chem. Soc. 2011, 133, 18122-18125. 60. S. Wesselbaum, T. vom Stein, J. Klankermayer, W. Leitner, Angew. Chem., Int. Ed. 2012, 51, 7499- 7502. 61. A. Boddien, F. Grtner, C. Federsel, P. Sponholz, D. Mellmann, R.Jackstell, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2011, 50, 6411-6414. 62. C. Ziebart, C. Federsel, P.Anbarasan, R. Jackstell, W. Baumann, A.Spannenberg, M. Beller, J. Am. Chem. Soc. 2012, 134, 20701-20704. 63. H. Meerwein, R. Schmidt, Justus Liebigs Ann. Chem. 1925, 444, 221. 64. R.V. Oppenauer, Recl. Trav. Chim. Pays-Bas 1937, 56, 137. 65. For selected examples see; a) T. Naota, H. Takaya, S.-I. Murahashi, Chem. Rev. 1998, 98, 2599; b) B. N. Chaudret, D. J. Cole-Hamilton, R.S. Nohr, G. Wilkinson, J. Chem. Soc., Dalton Trans. 1977, 1546; c) H. Imai, T. Nishiguchi, K. Fukuzumi, J. Org. Chem. 1976, 41, 665: d) A. Aranyos, G. Csjernyik, K.J. Szabo, J.-E. Bäckvall, Chem. Commun. 1999, 351; ibid, 2131; e) E. Mizushima, M. Yamaguchi, T. Yamagishi, J. Mol. Catal. A 1999, 148, 69; f) D. E. Linn, J. Halpern, J. Am. Chem. Soc. 1987, 109, 2969; g) P. A. Chaloner, M. A. Esteruelas, F. Joó, L. A. Oro, Homogeneous Hydrogenation, Kluwer Academic Publishers, Dordrecht, The Netherlands,1994, Ch. 3; h) E. Mizushima, M. Yamaguchi, T. Yamagishi, Chem. Lett. 1997, 237; i) Y. Nishibayashi, I. Takei, S. Uemura, M. Hidai, Organometallics 1999, 18, 2291; j) G. C. Jia, H. M. Lee, L. D. Williams, J. Organomet. Chem. 1997, 534, 173; k) J.-E. Bäckvall, J. Organomet. Chem. 2002, 652, 105. l) S. Bhaduri, K. Sharma, D. Mukesh, J. Chem. Soc., Dalton Trans. 1993, 12, 1191; m) C. S. Yi, Z. He, I. A. Guzei, Organometallics 2001, 20, 3641; n) O. Pàmies, J.-E. Bäckvall, Chem. Eur. J. 2001, 7, 5052. 66. For selected examples, see; a) S. Hashiguchi, A. Fujii, J. Takehara, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 1995, 117, 7562; b) J. X. Gao, T. Ikariya, R. Noyori, Organometallics 1996, 15, 1087; c) T. Ikariya, K. Murata, R. Noyori, Org. Biomol. Chem. 2006, 4, 393; d) R. Noyori, S. Hashiguchi, Acc. Chem. Res. 1997, 30, 97.
27 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
67. For selected examples, see; a) Y. Shvo, D. Czarkie, J. Organomet. Chem. 1986, 315, 25; b) Y. Blum, D. Czarkie, Y. Rahamim, Y. Shvo, Organometallics 1985, 4, 1459; c) B. L. Conley, M. K. Pennington- Boggio, E. Boz, T. J. Williams, Chem. Rev. 2010, 110, 2294; d) C. P. Casey, S. W. Singer, D. R. Powell, R. K. Hayashi, M. Kavana, J. Am. Chem. Soc. 2001, 123, 1090; e) N. Menashe, Y. Shvo, Organometallics 1991, 10, 3885; c) N. Menashe, E. Salant, Y. Shvo, J. Organomet. Chem. 1996, 514, 97; f) C. P. Casey, S. W. Singer, D. R. Powell, R. K. Hayashi, M. Kavana, J. Am. Chem. Soc. 2001, 123, 1090; g) N. Menashe, Y. Shvo, Organometallics 1991, 10, 3885; h) N. Menashe, E. Salant, Y. Shvo, J. Organomet. Chem. 1996, 514, 97. i) A. Landwehr, B. Dudle, T. Fox, O. Blacque, H. Berke, Chem. Eur. J. 2012, 18, 5701-5714. 68. a) M. Yamakawa, H. Ito, R. Noyori, J. Am. Chem. Soc. 2000, 122, 1466; b) H. P. Dijkstra, M. Albrecht, S. Medici, G. P. M. van Klink, G. van Koten, Adv. Synth. Catal. 2002, 344, 1135; c) T. Yamagishi, E. Mizushima, H. Sato, M. Yamachi, Chem. Lett. 1998, 1255. 69. a) N.C. Deno, H.J. Peterson, G. S. Saines, Chem. Rev. 1960, 60, 7; b) G. Brieger, T. J. Nestrick, Chem. Rev. 1974, 74, 567; c) R. A. W. Johnstone, A. H. Wilby, I. D. Entwistle, Chem. Rev. 1985, 85, 129; d) P. A. Chaloner, M.A. Esteruelas, F. Joó, L. A. Oro, Homogeneous hydrogenation. Kluwer Academic Publishers, Dordrecht, Boston, London, 1994, Ch. 3, p. 87 & 183; e) S. Gladiali, G. Mestroni, Transfer hydrogenations. In: M. Beller, C. Bolm (Eds.), Transition Metals for Organic Synthesis. Wiley-VCH, Weinheim, New York, Chichester, Brisbane, Singapore, Toronto, 1998, Ch. 3, p. 97. 70. S. Werkmeister, C. Bornschein, K. Junge, M. Beller, Chem. Eur. J. 2013, 19, 4437-4440. 71. S. Werkmeister, C. Bornschein, K. Junge, M. Beller, Eur. J. Org. Chem. 2013, 3671-3674. 72. a) Hydrosilylation, Advances in Silicon Science (Ed.: B. Marciniec), Springer, Berlin, 2009; b) A. K. Roy, Adv. Organomet. Chem. 2008, 55, 1; c) S. E. Gibson, M. Rudd, Adv. Synth. Catal. 2007, 349, 781; d) B. Marciniec, Appl. Organomet. Chem. 2000, 14, 527; e) R. Noyori, Asymmetric Catalysis in Organic Synthesis, Wiley, New York, 1994; f) I. Ojima, Catalytic Asymmetric Synthesis, VCH, New York, 1993; g) H. Brunner, W. Zettimeier, Handbook of Enantioselective Catalysis with Transition Metal Compounds, VCH, Weinheim, 1993; h) I. Ojima, The Chemistry of Organic Silicon Compounds (Eds.: S. Patai, Z. Rappoport), Wiley, New York, 1989, ch. 25. 73. a) M. Ochiai, H. Hashimoto, H. Tobita, Angew. Chem. 2007, 119, 8340; b) R. Calas, Pure Appl. Chem. 1966, 13, 61; c) A. J. Chalk, J. Organomet. Chem. 1970, 21, 207; d) R. J. P. Corriu, J. J. E. Moreau, M. Pataud-Sat, J. Organomet. Chem. 1982, 228, 301; e) B. Marciniec, Comprehensive Handbook on Hydrosilylation, Pergamon, Oxford, 1992. 74. a) R. J. P. Corriu, J. J. E. Moreau, M. Pataud-Sat, J. Organomet. Chem. 1982, 228, 301; a) T. Murai, T. Sakane, S. Kato, J. Org. Chem. 1990, 55, 449-453; b) T. Murai, T. Sakane, S. Kato, Tetrahedron Lett. 1985, 26, 5145-5148; c) A. M. Caporusso, N. Panziera, P. Petrici, E. Pitzalis, P. Salvadori, G. Vitulli, G. Martra, J. Mol. Catal. A 1999, 150, 275-285. 75. R. Calas, E. Frainnet, A. Bazouin, C. R. Hebd, Seances Acad. Sci. 1961, 252, 420-422. 76. a) T. Fuchigami, I. Igarashi, Jpn Patent Appl. JP11228579, 1999; b) A. Y. Khalimon, R. Simionescu, L. G. Kuzmina, J. A. K. Howard, G. I. Nikonov, Angew. Chem. 2008, 120, 7815; c) E. Peterson, A. Y. Khalimon, R. Simionescu, L. G. Kuzmina, J. A. K. Howard, G. I. Nikonov, J. Am. Chem. Soc. 2009,
28 Chapter 1 General Introduction
131, 908; d) J. Kim, Y. Kang, J. Lee, Y. K. Kong, M. S. Gong, S. O. Kang, J. Ko, Organometallics 2001, 20, 937; e) M. Tanabe, K. Osakada, Organometallics 2001, 20, 2118; f) H. Hashimoto, I. Aratani, C. Kabuto, M. Kira, Organometallics 2003, 22, 2199; g) T. Watanabe, H. Hashimoto, H. Tobita, J. Am. Chem. Soc. 2007, 128, 2176; 77. D. V. Gutsulyak, Georgii I. Nikonov, Angew. Chem. 2010, 122, 7715-7718. 78. a) L. Claisen, Ber. Dtsch. Chem. Ges. 1887, 20, 646-650; b) W. Tischtschenko, Chem. Zentralbl. 1906, 77, 1309-1311; c) T. Seki, T. Nakajo, M. Onaka, Chem. Lett. 2006, 35, 824-829; d) M. H. S. A. Hamid, P. A. Slatford, J. M. J. Williams, Adv. Synth. Catal. 2007, 349, 1555-1575; e) K. Ekoue-Kovi, C. Wolf, Chem. Eur. J. 2008, 14, 6302-6315; f) W. I. Dzik, L. J. Gooßen, Angew. Chem. 2011, 123, 11241- 11243. 79. Ullmann’s Encyclopadia of Industrial Chemistry, 6th edn., Wiley-VCH, Weinheim, 2002. 80. a) O. Kamm,W. F. Kamm, Org. Synth. Coll. Vol. 1, 1941,104; b) F. W. Swamer, C. R. Hauser, J. Am. Chem. Soc. 1946, 68, 2647-2649. 81. a) W. C. Child, H. Atkins, J. Am. Chem. Soc.1923, 45, 3013-3023; d) Y. Ogata, A. Kawasaki, Tetrahedron 1969, 25, 929-935; e) T. Ooi, T.Miura, K. Takaya, Tetrahedron Lett. 1999, 40, 7695– 7698; f) I. Simpura, V. Nevalainen, Tetrahedron 2001, 57, 9867-9872; g) T. Ooi, K. Ohmatsu, K. Sasaki, T. Miura, K. Maruoka, Tetrahedron Lett. 2003, 44, 3191– 3193; h) P. R. Stupp, J. Org. Chem. 1973, 38, 1433– 1434. 82. C. Tejel, M. A. Ciriano V. Passarelli, Chem. Eur. J. 2011, 17, 91–95. 83. W. I. Dzik, L. J. Gooßen, Angew. Chem. 2011, 123, 11241–11243, Y. Hoshimoto, M. Ohashi, S. Ogoshi, J. Am. Chem. Soc. 2011, 133, 4668-4671. 84. a) Zimmermann, S.; Sures, B. Environ. Sci. Pollut. Res. 2004, 11, 194-199; b) M. Schmid, S. Zimmermann, H. F. Krug, B. Sures, Environ. Int. 2007, 33, 385-390. 85. a) D. Heller, A. H. M. Vries, in Handbook of Homogeneous Hydrogenation, (Eds.: J. G. de Vries, C. J. Elsevier), Wiley-VCH: Weinheim, 2007; pp 1483-1516; b) Bartholomew, C. H. Appl. Catal. A 2001, 212, 17-60; c) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A Chem. 2003, 198, 317-341. 86. a) D. M. Heinekey, M. H. Voges, D. M. Barnhart, J. Am. Chem. Soc. 1996, 118, 10792–10802; b) C. Bianchini, A. Marchi, L. Marvelli, J. Am. Chem. Soc. 2011, 133, 8168-8178; c) M. Peruzzini, A. Romerosa, R. Rossi, A. Vacca, Organometallics 1995, 14, 3203-3215; c) D. Gusev, A. Llamazares, G. Artus, H. Jacobsen, H. Berke, Organometallics 1999, 18, 75-89. 87. a) A. Choualeb, O. Blacque, H. W. Schmalle, T. Fox, T. Hiltebrand, H. Berke, Eur. J. Inorg. Chem. 2007, 5246-5261; b) J. A. Gladysz, B. J. Boone, Angew. Chem., Int. Ed. Engl. 1997, 36, 550-583. 88. a) H. Berke, P. Burger, Comments Inorg.Chem. 1994, 16, 279-312; b) H. Jacobsen, H. Berke, in Recent Advances in Hydride Chemistry; (Ed.: R. Poli), Elsevier: Amsterdam, Holland, 2001; pp 89-116; c) A. Choualeb, E. Maccaroni, O. Blacque, H. W. Schmalle, H. Berke, Organometallics 2008, 27, 3474- 3481. 89. J. Chatt, S. Coffey, J. Chem. Soc. A 1969, 1963–1969. 90. M. L. Clarke, J. J. R. Frew, Ligand electronic effects in homogeneous catalysis using transition metal complexes of phosphine ligands; Organometallic Chemistry, 2009, 35, 19-46.
29 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
91. P. W. N. M. van Leeuwen, Homogeneous Catalysis: Understanding the Art, Kluwer Academic Publishers, The Netherlands, 2004. 92. M. Kranenburg, Y. E. M. van der Burgt, P. C. J. Kamer, P. W. N. M. van Leeuwen Organometallics 1995, 14, 3081-3089.
30 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
(Part of this work is published in a) B. Dudle, K. Rajesh, O. Blacque, H. Berke, Organometallics. 2011, 30, 29862992; b) B. Dudle, K. Rajesh, O. Blacque, H. Berke, J. Am. Chem. Soc. 2011, 133, 8168-8178)
2.1. Introduction
The area of homogeneous hydrogenations is dominated by platinum group metals.1 Due to
their toxicity, the precious metal components are to be removed after catalysis from the active
pharmaceutical ingredients in industrial processes.2 Also, due to their scarcity and high cost, these catalysts need to be recycled. Precious metal catalysis often suffers from reduction to the metals under hydrogenation conditions resulting in loss of catalytic activity.3 Being border to the precious metals in the periodic table, the element rhenium is expected to show
4 some of the precious metal’s character. This is revealed in their interaction with H2 and olefins.5 The most active catalytic homogeneous hydrogenation and related reactions consists of ruthenium generally phosphine or carbonyl or rhodium halide fragments.1,6 A rhenium nitrosyl fragment is isoelectronic with these fragments. These properties made us believe that suitable rhenium complexes could be efficient catalysts for hydrogenation and other related reactions.
Our research group has accumulated considerable experience in the realm of rhenium
2 nitrosyl chemistry, and we could recently demonstrated that trans-[ReH2(η - ethylene)(NO)(PR3)2] (R = i-pr, cy) complexes catalyze the hydrogenation of simple olefins and ketones,7 as well as hydrogen-related reactions, such as hydrosilylations, dehydrogenative silylations,8 and dehydrogenative aminoborane coupling reactions.9 Later during the progress
31 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes of our work, our group has come up with highly efficient hydrogenations of olefins using catalytic systems consisting of the above mentioned mono phosphine complexes along with suitable co-catalysts.10 We expected improvement of the catalytic performance of these complexes by forcing the phosphines into cis positions by applying chelating diphosphines.
The nitrosyl ligand can supports different oxidation states of metal centers often accompanied by different coordination modes.11,12 Furthermore it exerts a relatively strong trans-effect, leading to activation of the respective metal-ligand bonds. One of the latter influences is nitrosyl-substituted transition metal hydrides, in which the M−H bonds show increased hydridic character.11,12 Halogen ligands, like a bromide, are perfect π-donors and disposing them trans to the strong π-acceptor NO ligand would exert a stabilizing strong push pull π-interaction, which may lead to labilization of cis ligands.13
Diphosphines are often applied as ligands in combination with precious metals active for hydrogenation and related reductive reactions.14 The PMP angle (M = metal) in a metal diphosphine complex is termed as ‘bite angle’ which can have great influence on the steric and electronic properties and allows ligand tuning increasing the efficiency of the catalyst.15
A large bite angle of the ligands induces a distortion of the complexes mainly labilizing neighbouring ligands from an octahedral geometry that can lead to not only poorer orbital overlap between the metal and the bound atoms, but also to higher steric pressure.
The Xantphos family of ligands are often used in many catalytic transformations, particularly in hydroformilation reactions.15,16a 4,6-Bis(diphenylphosphino)-10,10- dimethylphenoxasilin (Sixantphos) (A),16a a diphosphine ligand belonging to the xantphos family of ligands bearing a silicon back bone with a large bite angle of 109°, the new derivative of Sixantphos ligand 4,6-Bis(diphenylphosphino)-10,10-diphenylphenoxasilin
(Sixantphos-Ph2) (B) as well as one with a sulfur atom in the backbone, 4,6-
32 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
bis(diphenylphosphino)phenoxathiin (Thixantphos) (C)16 were chosen as typical ligands to test our catalytic hypotheses (Figure 2.1).
Figure 2.1. Diphosphine ligands A-C (P∩P).
2.2. Results and Discussion
2.2.1. Preparation of [Re(diphosphine)Br2(NO)(CH3CN)]
17 The Re(II) complex [ReBr5(NO)][NEt4]2 (I), obtained as per reported literature procedure was found to be a suitable precursor for the preparation of Re(I) diphosphine complexes.18
The ligands Sixantphos (A) and Thixantphos (C) were also prepared according to literature procedures.16 Following a similar procedure for the preparation of A, we also prepared the related, but previously unknown Sixantphos-Ph2 compound (B).
The reduction of the Re(II) complex I in the presence of excess of the diphospines at a comparatively higher temperature of 200 °C in acetonitrile furnished the Re(I) complexes
Scheme 2.1. Preparation of Re(P∩P)Br2(NO)(CH3CN) complexes.
33 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
[Re(P∩P)(CH3CN)Br2(NO)] (IIA-IIC) in moderate yields (Scheme 2.1).
The reductions of the Re(II) to the Re(I) systems is facilitated by the oxidation of the excess of the respective ligand to RPRʹ2Br2 units. All these yellow coloured complexes IIA-
IIC were stable in the solid state under ambient conditions.
Various constitutional and conformational isomers are possible for an octahedral complex bearing a rigid bidentate ligand.19 The trans arrangement of the NO and one Br ligand is favoured due to a stabilizing strong “push-pull” π-interaction, making the
20 diphosphine ligand to stay cis position to Br/NO axis. The other two ligands CH3CN and Br are thus disposed cis i.e., trans to the diphosphine P atoms. Therefore, only one type of constitutional isomer of II could be formed. The backbone of a rigid large bite angle diphosphine ligand could by no means be forced into the P-Re-P plane. Therefore, the backbone of these ligands adopts either geometries close to Br (denoted as 1) or NO (denotes as 2) or even ‘twisted’ conformations. At least 1/2 conformers are expected for a ligand with planar back bone and these in complex IIA are relative to the Br/NO axis.
Suitable crystals of the complex IIA were obtained when pentane was layered on a dichloromethane solution of it. The X-ray crystal structure of IIA showed a much distorted- octahedral structure with a trans Br/NO disorder crystallizes as a mixture of diastereomers
IIA1 and IIA2 in a ratio of 86:14 and a cis diphosphine arrangement with respect to the
Br/NO axis (Figure 2.2). The preference for isomers 1 or 2 seemed to be controlled by steric effect excerted by the diphosphine. The isomer 1 is favoured apparently avoiding van der
Waals interaction of the Br ligand with one of the diphosphine phenyl groups with the closest
Br.....HPh. This effect causes a slight tilting of the Br/NO axis in the up isomer 2.
The bond lengths were found to be in the range of typical Re-Br, Re-NCCH3, Re-P and Re-NO bonds. However, the large bite- angle diphosphine causes steric congestion in the
34 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
Figure 2.2. Molecular structure of complex IIA Anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths: Re1- P1: 2.405(1); Re2-P2: 2.443(1); Re1-Br1: 2.6090(5); Re1-Br2a: 2.5591(6); Re1-N2: 2.126(3); Re1N1a: 1.717(4). Selected bond angles: P1Re1P2:97.01(3)°; N2Re1Br1: 84.46(9)°; P1Re1Br1: 89.40(3)°; P2Re1P2: 89.54(9)°.
P-Re-P plane, which is reflected in the compression of the N2-Re-Br2 angle down to
84.46(9). The P-Re-P angle in IIA was found to be 97.01(3)°, despite the fact that Sixantphos
(109°) possess substantially different natural bite angle.16,21 To explain this deviation, one has to take into account that the reported natural bite angles16 are determined by molecular dynamic simulations with a standard M-P distance of 2.3 Å. Since the actual Re-P distance in
IIA is about 2.42 Å, the natural bite angles are systematically too low for rhenium complexes. The “corrected natural bite angles” would thus be 100°. Even if we consider this corrected value as a reference, it should be noted that there would still be deviations, which presumably originate from a high degree of conformational flexibility of the diphosphine backbones, as well as from thermodynamically very strong Re-P bonds, for which an optimal orbital overlap between the phosphorus atoms and the rhenium center is important.
The IR spectra of IIA displayed characteristic ν(NO) bands at 1680 and 1686 cm-1. The 1H
NMR spectrum consists of broadened signals for the diphosphine and the CH3CN ligands.
Broad and overlapping signals were observed for bound and free CH3CN. This points to the presence of dissociation equilibria at room temperature. The 31P{1H} NMR spectra consisted
35 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
of two broad resonances, for which the coupling patterns could not be resolved. An
31 1 additional P{ H} NMR signal became observable, which reduced upon addition of CH3CN.
Similar spectroscopic observations were made for complex IIB and IIC, which could be
interpreted in terms of all these complexes [Re(A)Br2(NO)(CH3CN)] (IIA),
[Re(B)Br2(NO)(CH3CN)] (IIB) and [Re(C)Br2(NO)(CH3CN)] (IIC) in solution were in equilibria with their bromo-bridged dimers [Re2(A)2(Br)2(µ-Br)2(NO)2].2CH3CN (IIIA),
[Re(B)2(Br)2(µ-Br)2(NO].2CH3CN (IIIB) and [Re(C)2(Br)2(µ-Br)2(NO)].2CH3CN (IIIC), respectively, expelling acetonitrile which were also found in their lattice (Scheme 2.2).18
These processes were much less prominent in IIB and IIC. However, prolonged storage of
IIA in the solid state for many days led this complex to stay as dinuclear complex IIIA. This equilibrium could be shifted considerably to the CH3CN complex side by heating with
CH3CN.
Scheme 2.2. Bridging and splitting equilibria between complexes IIA-IIC and IIIA-IIIC respectively.
The CH3CN dissociation is expected to be promoted by “steric pressure” imposed on this ligand by the large-bite-angle diphosphines. The broadness of the 1H and the 31P{1H}
NMR spectra and the absence of the expected coupling pattern of the 31P{1H} NMR signals can be explained by assuming dynamics with exchange of the inequivalent 31P nuclei at a rate in the range of the NMR time scale. This exchange might proceed via either formation of μ2-
Br intermediates, which is assumed to be cleaved randomly, or via the formation of a transient unsaturated trigonal bipyramidal intermediate, which can rebind the freed CH3CN at
36 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
NO P Br NO Br O Re P NCCH3 P Br P Br O Re Re O . 2 CH3CN + CH CN P Br P IIA-IIC 3 Br NO - CH3CN IIIA-IIIC
NO NO P Br Re P O O Re P P Br Br Br + CH3CN NO - CH3CN P NO O Re Br P NCCH3 P O Re Br P Br Br
Scheme 2.3. Mono- and dinuclear racemization pathways of IIA-C for the Br/CH3CN exchange.
both sides of the Br ligand with equal probability (Scheme 2.3). To gain further insight into the dynamics, we recorded the 31P{1H} spectra of IIA at low temperature, thus slowing down the exchange processes. The room-temperature spectrum of a solution of IIA in CDCl3 consisted of two broad signals at 1.0 and -4.0 ppm assigned to IIA and a sharp signal at 26.1 ppm assigned to IIIA. Warming the sample to 320 K leads to a significant broadening of the signal of IIIA at 26.1 ppm. At the same time the two signals of IIA collapsed into a
enantiomers
Figure 2.3. General sketch for isomerism in Re(P∩P)XY(Br)(NO) complexes
37 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
coalescing signal at -5.2 ppm. Cooling a sample of IIA to 240 K led to a sharp signal for IIIA at 26.0 ppm (0.08 P) and to four sets of doublets for IIA at 4.7 and 0.1 ppm (0.10 P), at 0.3 and -3.4 ppm (2JPP = 10Hz, 0.66 P), at -3.7 and -11.4 ppm (0.04 P), and at -7.2 and -14.1 ppm (0.12 P). From these results we can conclude that different conformers are present in solution. Four conformers are distinguishable with prevailing amounts of IIA1, which would be in accord with the results of the crystallographic analysis of this complex. Minor signals were assigned to IIA2, IIA(twisted-a) and IIA (twisted-b) (Figure 2.3). At room temperature these conformers are in fast exchange on the NMR time scale. An additional exchange is observed between IIA and IIIA at elevated temperatures, indicating that the isomerization pathway via the μ2-Br dimers is operative in this case (Scheme 3).
2.2.2. Reaction of IIA-IIB and IIIA with Et3SiH
Analogous complexes of type II or III bearing other diphosphines upon reaction with
Et3SiH dihydride silyl complexes [ReBr(H)2(SiEt3)(NO)(P∩P)] (IV), for which a pentagonal- bipyramidal structure is assumed with the hydrides, the silyl moiety, and the diphosphine all in the pentagonal plane, in close analogy to one of the structurally fully characterized
17 [ReBr(H)2(SiMe3)(NO)(P∩P)], P∩P = 1,1’-bis(diphenylphosphino)ferrocene. When this reaction of Et3SiH with IIA or IIIA or with a mixture of both was carried out, mixture of
NO P NO NO Br P P H + Et3SiH H O - Et3SiH Re O Re O Re SiEt - Et SiBr 3 P 3 + Et3SiH NCCH3 P P H Br - CH3CN Br Br IIA VA IVA
NO P H 1/n O Re P Br n
Scheme 2.4. Reaction of IIA with Et3SiH.
38 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
products were observed, which could not be separated. According to NMR studies, we
propose that at least one of the products has a silyl dihydride structure related to IV. Like few of the analogous complexes of IV bearing other diphosphines,18 this complex was found to be stable in solution, but only in the presence of the silane. Attempts to isolate it led to the formation of a brick red precipitate similar to the cases of attempted isolation of other complexes IV,18 from which we concluded that IVA is in equilibrium with the 16e complex
2 [Re(A)HBr(NO)] (V) and Et3SiH, which subsequently may oligomerize to μ -(H)2 dimers, trimers, or even higher oligomers composed of VA units (Scheme 2.4).18 Similar oligomerizations of isoelectronic intermediates were reported to be formed from Crabtree’s
22 [Ir(diene)(PCy3)(pyridine)][PF6] catalysts in the presence of H2 or the [Re(H)7(PPh3)2] complex at elevated temperatures.23
2 2.2.3. Preparation of [Re(oCPPh-P∩P)(η -ethylene)Br(NO)] (VII)
Then, we thought to carry out the reaction of IIA or IIIA with excess of Et3SiH in the presence of ethylene as an additional auxiliary. Ethylene was considered an ideal ligand for the stabilization of the 16e species V because of its small size and appropriate binding capabilities to electron-rich Re(I) centers.24 Additionally, in view of the application of the expected [Re)(P∩P)BrH(η2-ethylene)(NO)] (VI) (Scheme 2.6) compound to function as a precatalyst in hydrogenation catalyzes, the ethylene ligand was anticipated to be initially hydrogenated to ethane, generating the desired highly reactive intermediate V, which could then drive the catalytic cycle.
The reaction of analogous complexes of the type II or III bearing diphosphine ligands proceeded smoothly in presence of an excess of Et3SiH and 2 bar of ethylene to yield the isolable [Re(P∩P)BrH(η2-ethylene)(NO)] (IV) complexes.18 However, this reaction using complexes IIA or IIIA of mixture of both and IIB or IIIB or a mixture of both unexpectedly
39 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Et3SiH (excess)
(2 bar) 70 oC, 6 h CH2Cl2
Scheme 2.5. Reaction of IIA and IIB with Et3SiH and ethylene to from the ortho metallated rhenacycles VIIA and VIIB respectively.
2 furnished two ortho metalated isomers in either case [Re(oCPPh-A)(η -ethylene)Br(NO)]
2 (VIIA1 and VIIA2) in a ration of 1:0.4 as well as [Re(oCPPh-B)(η -ethylene)Br (NO)]
(VIIB1 and VIIB2) in a ration of 1:0.3 respectively, which were isolated by column
chromatography under normal conditions (Scheme 2.5). Single crystals of all these
compounds could be obtained suitable for their structural characterization by X-ray
diffraction (Figure 2.4). All these isomers were found to be stable in both solution and solid
state. We propose the ortho metalation to proceed via the unsaturated 16e species
[Re(A)BrH(NO)] (VA). This species bearing a comparatively large bite angle diphosphine can apparently not sufficiently be stabilized by the binding of ethylene or CH3CN. These
40 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
Scheme 2.6. Transformation of IVA and IVB into IVA and IVB.
auxiliary ligands can dissociate and the remaining unsaturated species are reactive enough to activate the oC-H bond of one Sixanphos phenyl units forming the [Re(oCPPh-
P∩P)Br(H2)(NO)] (VIIIA and VIIIB) species, which in turn can undergo a H2/ethylene ligand exchange forming, via the [Re(oCPPh-P∩P)Br(NO)] complexes (IXA and IXB), VIIA and VIIB isomers, respectively (Scheme 2.6).
VIIA1; Selected bond distances: C1Re1: 2.194(3), VIIA2; Selected bond distances: C1Re1: 2.199(2), C2Re1: 2.225(3), C22Re1: 2.165(3), N1Re1: 1.819(3), C2Re1: 2.235(2), C3Re1: 2.177(2), N1Re1: 1.747(2), P1Re1: 2.5039(8), P2Re1: 2.5605(8), Br1Re1: P1Re1: 2.5017(6), P2Re1: 2.5741(5), Br1Re1: 2.5816(3); Selected bong angles: P1Re1P2: 94.41(2), 2.5912(2); Selected bong angles: P1Re1P2: 91.73(2), C1Re1C2: 36.9(1), N1Re1Br1: 174.44(9), P1Re1Br1: C1Re1C2: 37.11(8), N1Re1Br1: 179.01(6), P1Re1Br1: 87.18(2), P2Re1Br1: 91.70(2), C22ReP1: 64.61(8). 90.84(1), P2Re1Br1: 85.99(1), C3ReP1: 64.67(6).
41 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
VIIB1; Selected bond distances: C1Re1: 2.228(8), VIIB2; Selected bond distances: C1Re1: 2.183(8), C2Re1: 2.219(6), C8Re1: 2.170(5), N1Re1: 1.842(4), C2Re1: 2.238(6), C8Re1: 2.174(5), N1Re1: 1.758(4), P1Re1: 2.487(1), P2Re1: 2.576(2), Br1Re1: 2.5767(6); P1Re1: 2.485(1), P2Re1: 2.546(1), Br1Re1: 2.589(1); Selected bong angles: P1Re1P2: 93.19(4), C1Re1C2: Selected bong angles: P1Re1P2: 92.91(4), C1Re1C2: 33.7(2), N1Re1Br1: 176.2(1), P1Re1Br1: 92.33(3), 37.0(2), N1Re1Br1: 178.3(2), P1Re1Br1: 91.26(3), P2Re1Br1: 92.33(3), C8ReP1: 64.6(1). P2Re1Br1: 85.82(3), C8ReP1: 64.5(1).
Figure 2.4. Molecular structure of VIIA1, VIIA2, VIIB1 and VIIB2. Anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity.
2.2.4. Hydrogenation of Olefins Using the Complexes of the Type VII
Complexes VIIA and VIIB turned out to be (pre)catalysts for the hydrogenation of olefins showing activities comparable to Wilkinson or Osborn-type Rh catalysts.25,10a To explore the catalytic capabilities VIIA and VIIB in the catalytic hydrogenations of olefins, we employed a press gas flow controller for quantitative kinetic monitoring. Hydrogenation of styrene under the addition of various co-catalysts and a screening of solvents under a H2 pressure of
10 bar at 80 °C is shown in Table 2.1. The reaction rates were found to be drastically improved when the reactions were carried out in a suitable solvent. Slower reaction led to the polymerization of styrene. Toluene was found to be an ideal choice among the tested solvents. Also, addition of Et3SiH as a co-catalyst was found not only to increase the rate of reaction, but also to stabilize the species in order not to decrease the activity until the substrate was fully consumed. The catalytic performance of the two isomers, VIIA1 and
42 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
a Table 2.1. Hydrogenation of styrene using catalyst X under 10 bar H2 at 80 °C.
Re(I) cat./Et3SiH (0-115 equiv. w. r.to Re(I) cat.) Ph o 10 bar H2, 80 C, With or without solvent Ph TOF: up to 2960 h-1; TON: up to 24135 Yield : up to 100%
-1 Et3SiH TOF (h ) TON Conversion Entry Catalyst Solvent b Time (h) (Equiv.) (1st h) Overall (%)
1 VIIA1 - - 929 148 29 4308 18 197 29 5703 24 2 VIIA2 - - 1157 155 86 13324 55
3 VIIA2 Toluene - 2207 428 15 6418 27 4 VIIA1 Toluene 115 2414 2414 10 24135 100 5 VIIA2 Toluene 115 2961 2961 8 24135 100 6 VIIA2 Neat 115 1518 1518 16 24135 100 7 VIIA2 Toluene 575 1707 1707 14 24135 100 8 VIIA2 Toluene 23 2379 2379 10 24135 100 9 VIIA2 THF 115 2601 1006 24 24135 100 10 VIIA2 DCM 115 2242 2194 11 24135 100 a5 mg of catalyst was used with 15 mL of styrene and 10 mL of solvent, carried out using a Büchi pressgas flow controller and monitor. bWith respect to catalyst.
VIIA2, in hydrogenations was found to be somewhat different on a quantitative scale, pointing to the fact that under catalytic conditions a 1 ↔ 2 isomerization14 does not take place. In absolute numbers, however, these differences appeared to be small (Table 2.1).
TOFs of up to 2960 and TONs of more than 24000 could be realized under these relatively mild conditions in the hydrogenation of styrene.
Then, we applied this hydrogenation strategy for a variety of substrates including the sterically demanding disubstituted substrates cyclohexene, α-methylstyrene, and dimethyl itaconate using catalysts VIIA1 and VIIIA2 along with Et3SiH as a co-catalyst in toluene
(Table 2.2). Using VIIA, an alkyne, like phenyl acetylene could also be hydrogenated
completely to ethyl benzene, however, phenyl acetylene was found to be relatively strongly
43 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Table 2.2. Hydrogenation of various olefins and phenylacetylene using catalyst VII.a
R R'' R R'' Re(I) cat./Et3SiH (115-125 equiv.)
10-50 bar H , 80-140 oC R' H 2 R' H Toluene TOF: up to 14000 h-1 R, R', R'' =alkyl, aryl, C(O)OMe, CH2C(O))Me TON: up to 31500
Entry Olefin Cat. Temp. TOF (h-1) Time TON Conv. (h) (%) (1st h) Overall
1 1-Hexene VIIA1 80 4867 3139 2 6278 28 2 1-Hexene VIIA1 120 4145 7796 0.5 3898 17 - 2 4405 20 3 1-Hexene VIIA2 60 2818 2834 2.5 7086 32 4 1-Hexene VIIA2 80 4120 3375 1.5 5063 23 5 1-Hexene VIIA2 120 3644 6126 0.5 3063 14 2601 1.5 3902 18 6 Cyclohexene VIIA1 120 601 372 17 6327 23 7 Cyclohexene VIIA2 120 1231 566 18 10190 37 8 Styrene VIIA1 120 6996 6996 3.75 24135 100 9 Styrene VIIA2 120 8048 8048 3 24135 100 10 Styrene VIIA 120 7485 7485 3.25 24135 100 11 Styrene VIIB 120 8200 8200 3 27402 100 12 α-Methylstyrene VIIA1 120 1820 1820 11 20025 100 13 α-Methylstyrene VIIA2 120 1940 1940 10.3 20025 100 14b Dimethyl itaconate VIIA1 140 4038 595 41 24400 73 15 Dimethyl itaconate VIIA2 140 4915 500 63 31492 95 16b Phenylacetylene VIIA 140 - 232c 10 2322 100 17b Phenylacetylene VIIA 140 1774/ - 36 5239/ 12.5/ 1067d 2695d 6.4d
a Unless mentioned, 5 mg of catalyst, 0.1 mL of Et3SiH and 2/3(substrate in mL) mL of toluene under 10 bar b of H2 applied using a Büchi pressgas flow controller and monitor. 50 bar of H2, yields and conversions by GC/MS based on the consumption of the substrate. c3.6 mg of catalyst and 1 mL of phenylacetylene (0.043 mol%). dRatio between styrene and ethylbenzene.
coordinating to the rhenium centre, which made the styrene to undergo hydrogenation comparatively in a much slower rate, until the whole alkyne was consumed.
44 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
2.2.5. Kinetics and Mechanism of the Hydrogenations Using Complexes of the Type VII
However, VIIA and VIIB were not efficient in hydrogenation of 1-hexene in comparison to that of styrene. Concomitant to the hydrogenation process, an intervening isomerization reaction, transforming 1-hexene into Z/E-2-hexene, which was found to proceed at a rate comparable to that of hydrogenations under catalytic conditions (80 °C, 10 bar H2), and these
internal olefins are hydrogenated at a much slower pace.
The reaction rates were found to be zeroth order in olefin concentration. This implies
relatively strong olefin binding and a high olefin affinity to the rhenium center, which leads
already at very low olefin concentration to quantitative olefin saturation of the catalyst.
Probing of the kinetic isotope effect (KIE) with H2 and D2 revealed a kH2/kD2 value of 0.46
which suggest a late transition state of reductive elimination process as the rate limiting step
(Table 2.3, entries 13 and 14). When complex VIIA was reacted with 2 bar of D2 at 90 °C in benzene, we observed apart from ethane analyzed by 1H NMR spectroscopy, the incorporation of D in the ortho carbon atom of the phenyl group (7.15 ppm) on P atom as well as Re-D species (-2.62 ppm) as analyzed by 2H NMR spectroscopy (Scheme 2.7).
Storage of this sample for a week led to the formation of single crystals suitable for X-ray diffraction studies which revealed the dinuclear H (D) bridged complex XA (Figure 2.5).
Figure 2.5. Molecular structure of complex XA. Anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms, phenyl groups on P atoms and solvent molecules are omitted for clarity.
45 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
This indicated that the complex VII underwent hydrogenolysis of the Re-C bond to form subsequently the species VI. Insertion of ethylene into the Re-H bond is supposed to give rise to the coordinatively unsaturated 16e ethyl species XI. Dihydrogen coordination and
Scheme 2.7. Reaction of VIIA1 with D2.
Scheme 2.8. Proposed catalytic cycle for the hydrogenation of olefins using VII.
46 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
oxidative addition to this complex is followed by reductive elimination of the alkane, would
regenerates the coordinatively unsaturated 16e complex V which has also be considered as the active species of the catalytic reaction course (Scheme 2.8). This species, would dimerize in the absence of ethylene to give the stable 18e XA. In the presence of olefin, V would form
the species VI, which is capable of undergoing ortho metallation reaction to form the rhenacycles. Now, the crucial role of Et3SiH as a co-catalyst is presumed to lie in the cleavage of the dinuclear complex X forming the active species V, via the
silyldihydridorhenium species IV (Scheme 2.7).
The observed inverse kinetic isotopic effect indicated a late transition state as rate
limiting. This could be attributed to either the reductive elimination of the alkane from
species XII or that of the phenyl group from the ortho metallated dihydride species VIII,
both to form the active species V (Scheme 2.8). However, the former step as rate limiting can
be ruled out when the same active species formed in the hydrogenation reactions using
complex IIIA, where the CH3CN present in the catalyst would suppress the ortho metallation
pathways, showed a normal DKIE in the hydrogenation of styrene (vide infra).
2.2.6. Hydrogenation of Olefins and their Mechanism Using Complexes of the Type II and IIIA
Then the strategy of hydrogenation reaction of styrene was tested using the complexes IIB-
IIC and IIIA or a mixture of both IIA and IIIA (Table 2.3; for a comparison, various catalytic systems are also shown). It is worth mentioning that either IIA, IIIA or a mixture of both showed the same activity for hydrogenation and related reactions indicating that the same active species is formed and operative irrespective of the ratio of II and III. Since most of the reactions are carried out after a prolonged storage of these complexes leading IIA to
form IIIA, we will be using the notation of IIIA for the reactions hereafter. All these
47 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes complexes in the absence of a co-catalyst were found to be active catalysts for these hydrogenation reactions, but were comparatively slow leading to partial polymerization of
a Table 2.3. Hydrogenation of styrene using various catalysts under 10 bar H2 at 120 °C.
Re(I) cat./Et3SiH (0-142 equiv. w. r.to Re(I) cat.) Ph 10 bar H , 120 oC, Toluene Ph 2 Yield : up to 100% TOF: up to 14070 h-1; TON: up to 24960
Entry Catalyst Co-catalystb TOF (h-1) TON Conversion (%)
1 IIIA - 150 750 -c,d
2 IIIA - 1063 2126 -c,e, - 3 IIIA 1700 1700 85f
4 IIIA Et3SiH 14073 26629 100
5 IIB Et3SiH 11701 29896 100
6 VII Et3SiH 7540 23096 100
g 7 VIIA Et3SiH + 10916 24135 100 CH3CN
8 VIIA Et3N 9249 24135 100
9 VIIA Et3SiH 16414 24135 100
10 IIIA Et3SiH 9490 26629 100
h i 11 XIIIA Et3SiH 8590 28957 100
c i 12 XIVD Et3SiH 2710 21181 100
j 13 IIIA Et3SiH 9490 26629 100
j 14 VIIA Et3SiH 16414 25135 100
aUnless mentioned, 5 mg of catalyst was used with 15 mL of styrene (0.00472-0.00335 mol%). b10 mL of solvent, carried out using a Büchi press gas flow controller and monitor. b0.1 mL of Et3SiH (115-126 equiv. c d e f with respect to catalyst). Partially polymerized. Run for 5 h. 30 bar H2 and run for 2 h. 0.05 mol% of g catalyst under 50 bar H2 at 90 °C run for 1 h. 10 equiv. of CH3CN with respect to catalyst was added. hUntill 60% conversion; decrease in activity was observed after this leading to completion of the reaction in 15 h. iFor the first 1 h; decrease in activity was observed from the beginning itself leading to completion of j the reaction in 19 h. Reaction with 10 bar of D2.
48 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
styrene, unless higher catalyst loadings were adopted, when compared to reactions applying
the addition of Et3SiH as an activating co-catalyst (Table 2.3, entries 1-3). The mechanism
which is thought to be operative in the hydrogenation using IIIA as potent catalysts in the absence of a co-catalyst will be discussed in Chapter 4.
However, the active species in the catalytic hydrogenation reactions using IIIA, IIB-
IIC with Et3SiH as co-catalyst is undoubtedly species V, since all the components included in the reaction of formation of VII and their catalytic hydrogenations are present here. In addition, one has to take into consideration the availability of stoichiometric quantities of
CH3CN and Et3SiBr. Surprisingly, these reactions were found to be much more active in the hydrogenation of styrene. Also, a small DKIE value of 1.47 was observed replacing H2 by D2.
Addition of 10 equiv. of CH3CN in the hydrogenation system consisting of VII was found to increase the rate of the reaction. Even the addition of a 10 equiv. of triethylamine with respect to VIIA was found to increase the rate of reaction. This amine was added assuming that the
CH3CN, which is part of the complexes III would undergo hydrogenation to form this substituted amine. The ability of this complex to hydrogenate nitriles was found under forcing conditions giving higher substituted amines (Chapter 3), which would stabilize the active species V.
From all these observations, one can conclude that the presence of stoichiometric amounts of CH3CN in the reaction medium would stabilize the active species forming a resting state so that the latter would not undergo oxidative addition reactions leading to ortho metallation, where the reverse, reductive elimination of a phenyl group expected to be slow and the rate limiting. Thus, a catalytic cycle without the ortho metallation steps, would operate for the hydrogenation reactions using III. Homolytic splitting of H2 by oxidative addition is expected to be rate limiting here.
49 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
A comparison of the activity of all these catalysts IIIA, IIB and IIC as well as VIIA,
VIIB and VIIA+CH3CN systems, all along with Et3SiH as a co-catalyst carried out under 10
bar of H2 pressure at a temperature of 120 °C in toluene as a solvent are shown as kinetic plot
with monitoring of the H2 consumption in Figure 2.6.
IIIA IIB
IIC VIIA VIIB VIIA + CH3CN
Figure 2.6. Comparison of the H2 consumption of various catalyses for hydrogenation of styrene under constant
H2 pressure of 10 bar at 120 °C in toluene.
2.2.7. Preparation of [Re(POP)I2(NO)] (POP = A) and Catalytic Hydrogenation of Styrene
Then, we thought to prepare the diiodo complex analogous to IIA of IIIA to study the effect
of the halide influence in these catalyses. We reacted IIIA with NaI in acetone or dichlorometane at a temperature of 50 °C (Scheme 2.9). The 31P NMR spectrum of the isolated compound showed a new singlet resonance at 24.2 ppm, but the 1H NMR spectrum did not show any CH3CN signal. Single crystals suitable for X-ray diffraction were obtained when benzene was layered over a dichloromethane solution of this compound, which was
50 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
structurally analyzed to be the diiodo complex XIIIA, where the Sixantphos ligand of this
compound was found coordinated in a tridentate fashion with the O atom also involved in the
NO Ph Ph P I NO Br Re I P Br P NaI (5 equiv.) Ph P O Re Re O . 2 CH CN 3 Ph O P Br P Acetone or CH2Cl2 Br NO 50 oC, 3 h
IIIA 96% XIIIA Si
Scheme 2.9. Reaction of IIIA with NaI.
Figure 2.7. Molecular structure of complex XIIIA Anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths: Re1-I1: 2.7952(2), Re1-I2: 2.7976(2), Re1-O1: 2.221(2).
bonding to the rhenium centre trans to the NO ligand and the two iodides were found disposed trans to the diphosphine ligand (Figure 2.7).
The hydrogenation reaction of styrene using this complex XIIIA was found to be inferior in activity when compared to that of IIIA. However, the strategy to add Et3SiH as a co-catalyst was found to drastically increase the efficiency of this reaction, but still was inferior to the reaction using IIIA under the same conditions otherwise (Table 2.3, entry 11).
51 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
2.2.8. Preparation of [Re(DBFmonophos)(CH3CN)2Br2(NO)] (XIVD) and Catalytic Hydrogenation of Styrene
As described in Chapter 4, the complexes IIIA and XIIIA were found to be highly active for the hydrogenation of imines, and as a step on the elucidation of their catalytic reaction courses, we later prepared the monophosphine complex [Re(D)(CH3CN)2Br2(NO)]
NO O H3CCN Br THF Re + NO H CCN NCCH O o Ph P NCCH3 3 3 Ph P 130 C, 6 h 2 Br 2 Re 78% DBFmonophos H3CCN Br XV D Br XIVD
Scheme 2.10. Preparation of [Re(DBFmonophos)(CH3CN)2Br2(NO)].
Figure 2.8. Molecular structure of complex XIVD Anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bnd lengths: Re1P1: 2.40005(8), Re1Br1: 2.6037(4), Re1N2: 2.070(3); Re1N3: 2.068(3).
26 (XIVD) bearing the ligand DBFmonophos (D) by the reaction of mer-[Re(CH3CN)3
27 Br2(NO)] (XV) with D in THF (Scheme 2.10). Single crystals suitable for X-ray diffraction analysis was obtained when pentane was layered on a dichloromethane solution of this
52 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
compound allowing it to be characterized structurally by single crystal X-ray diffraction
(Figure 2.8).
Surprisingly, though far less efficient as catalyst when compared to the diphosphines
discussed, this monophosphine rhenium complex XIVD was also found to be active in the hydrogenation of styrene along with Et3SiH as a co-catalyst (Table 2.3, entry 12).
2.3. Conclusion
Large bite angle diphosphine nitrosyl rhenium(I) complexes of the type
[Re(P∩P)(CH3CN)Br2(NO)] (II) and [Re2(A)2(Br)2(µ-Br)2(NO)2].2CH3CN (IIIA) could be prepared. On attempts to prepare complexes of the type [Re(P∩P)(ɳ2-ethylene)HBr(NO)]
(VI), where the H and ethylene cis to each other, an unusual reactivity driven them to ortho metallation of one of the phenyl groups of the diphosphine ligand. This led to the formation
2 of four membered rhenacycles of the type [Re(oCPPh-P∩P)(η -ethylene)Br(NO)] (VII). These complexes when reacted with H2 underwent hydrogenolysis of the Re-CPh bond leading to the
transient formation of the desired complexes which were highly reactive to hydrogenate the
olefins generating the active species [Re(P∩P)HBr(NO)] (V). All the complexes II, III and
VII were found to be active catalysts for the hydrogenation of olefins, for instance, TOFs of
-1 up to 14000 h could be achieved under a H2 pressure of 10 bar at 120 °C, without the loss of
activity, and that too with a relatively high quantity of 15 mL of styrene giving rise to
complete conversion to ethyl benzene. A catalytic cycle operating for these hydrogenation
reactions using VII and II, the latter in the presence of Et3SiH as co-catayst is elucidated. The
activity could be drastically improved by the addition of Et3SiH as co-catalyst in reactions
using VII. Oxidative addition by heterolytic splitting of H2 is assumed to be the rate limiting
step for hydrogenation reactions using complexes II and IIIA. However, the reductive
elimination from the ortho metallated species VIII is proposed to be the rate limiting step in
53 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes the hydrogenation reactions using VII. The mechanism of the hydrogenation reactions using
II or III in the absence of any co-catalyst will be discussed in Chapter 4.
2.4. Experimental Section
General Procedures
All manipulations were carried out under an atmosphere of dry nitrogen using standard Schlenk techniques or in a glove box (M. Braun 150B-G-II) filled with dry nitrogen. Solvents were freshly distilled under N2 by employing standard procedures and were degassed by pump freeze-thaw cycles prior to use.
17 16b 16 [ReBr5(NO)][NEt4]2 (I) , Sixantphos (A) , and Thixantphos (C) were prepared according to reported procedures. Triethylsilane was purchased from abcr speciality chemicals and used without further purification.
1H NMR, 13C{1H} NMR and 31P{1H} NMR data were recorded on a Bruker-500 MHz spectrometer.
Chemical shifts are expressed in parts per million (ppm) referenced to the deuterated solvent used. All chemical
31 1 shifts for the P{ H} data are reported downfield in ppm relative to external 85% H3PO4 at 0.0 ppm. Signal patterns are reported as follows: s, singlet; d, doublet; t, triplet; td, doublet of triplet; dt, triplet of doublet, m, multiplet. IR spectra were obtained by using ATR methods with a Bio-Rad FTS-45 FTIR spectrometer. Signal intensities in the spectra are reported as follows: s, strong; m, medium; w, weak. Elemental microanalysis was carried out with Leco CHNS-932 analyser.
Preparation of Sixantphos-PPh2 (B)
An analogues procedure for the synthesis of sixantphos (A) was adopted16a.
10,10-Diphenylphenoxasilin
At room temperature a solution of diphenyl ether (1.0 g, 5.88 mmol) in THF (5 mL) was added dropwise to a mixture of 2.5 M n-butyllithium in hexanes (5.2 mL, 13.0 mmol) and TMEDA (2.1 mL, 13.0 mmol). When the phenyl ether addition was complete, the reaction mixture was stirred for 16 h. The reaction mass was diluted with ether (5 mL) and to this a solution of dichlorodiphenylsilane (1.22 mL, 5.88 mmol) in ether (10 mL) was added over 40 minutes and the mass was stirred for another 16 h and then water (10 mL) was added. The mixture was stirred for 2 h. The organic layer was separated and the aqueous layer was extracted with ether (15 mL). The combined organic layers were washed with 10 mL of water. The combined organic layers were dried
54 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
over MgSO4. The solvent was removed on a rotary evaporaor. The semisolid oil was crystallized form 2- propanol, resulting in white crystals. Yield: 0.55 g (55%); IR (KBr, cm-1): 3064 (w), 3048 (w), 3006 (w), 1956
(w), 1816 (w), 1618 (m), 1588 (s), 1570 (s), 1481 (w), 1460 (s), 1420 (s), 1429 (s), 1300 (s), 1266 (s), 1216 (s),
1183 (m), 1156 (m), 1126 (s), 1110 (s), 1075 (m), 1026 (m), 997 (m), 885 (m); 1H NMR (500 MHz, CDCl3): δ
7.19 (td, 2H, J = 12, 1.5 Hz), 7.29 (d, 2H, J = 13 Hz), 7.40-7.43 (m, 4H), 7.45-7.49 (m, 2H), 7.50-7.53 (m, 2H),
7.62-7.66 (m, J = 6H); 13C NMR (125 MHz, CDCl3): 116, 118.3, 122.8, 128.1, 130.0, 131.7, 134.0, 135.4,
135.9, 160.4.
4,6-Bis(diphenylphosphino)-10,10-diphenylphenoxasilin (Sixantphos-Ph2) (B)
Ph Ph Si
O
PPh2 PPh2 B
At room temperature 1.4 M sec-butyllithium in 98/2 cyclohexane/hexane (7.2 mL, 8.55mmol) was added dropwise to a stirred solution of 10,10-Diphenylphenoxasilin (1 g, 2.85 mmol) and TMEDA (1. 3 mL, 8.55 mmol) in dry ether (40 mL). When all sec-butyllithium was added, the reaction mixture was stirred for 16 h.
Then a solution of chlorodiphenylphosphine (1.6 mL, 8.55 mmol) in hexane (15 mL) was added, and the reaction mixture was stirred for another 16 h. The solvent was removed in vacuo. The resulting solid oil was dissolved in dichloromethane (20 mL), washed with water (2 × 10 mL) and dried over MgSO4 and the solvent removed on a rotary evaporator. The resulting oil was washed with hexanes and crystallized from 1-propanol to get the title compound as white powder. Yield: 1.17 g (57%); IR (KBr, cm-1): 3067 (w), 3051 (w), 3025 (w),
1956 (w), 1889 (w), 1825 (w), 1618 (w), 1578 (m), 1570 (m), 1476 (m), 1432 (s), 1397 (s), 1368 (s), 1301 (w),
1283 (w), 1235 (s), 1200 (s), 1155 (w), 111 (m),m 1066 (w), 1026 (w), 997 (w), 885 (w); 1H NMR (500 MHz,
CDCl3): δ 6.75 (dq, 2H, J = 7.5, 2.0 Hz), 6.90 (t, 2H, J = 7 Hz), 7.10-7.19 (m, 20 H), 7.31 (t, 4H, J = 7.5 Hz),
7.36 (d, 2H, J = 7.5 Hz), 7.45 (dd, 2H, J = 7.5, 1.8 Hz), 7.52 (dd, 4H, J = 8, 1.5 Hz); 31P {1H} NMR (201 MHz,
13 CDCl3): δ -17.7 (s), C NMR (125 MHz, CDCl3): δ 116.4, 123.8, 128.7 (t, J = 3.4 Hz), 128.8, 130.6, 134.5,
134.7 (t, J = 10.6 Hz), 136.6, 136.7, 137.6, 138.6 (t, J = 6.6 Hz); Anal. (%). Calc for C48H36OP2Si: C, 80.20; H,
5.05. Found: C, 79.98; H, 4.94.
Preparation of [Re(A)(CH3CN)Br2NO] (IIA), [Re(B)(CH3CN)Br2NO] (IIB) and [Re(C)(CH3CN)Br2NO] (IIC) Compound IIA
55 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
[ReBr5(NO)] [NEt4]2 (I) (1 g, 1.14 mmol) and sixantphos (A) (0.85 g, 1.425 mmol) in acetonitrile (12 mL) was taken in an autoclave and heated to 200 oC for 4 h. Cooled to room temperature, filtered to get an yellow solid.
It is washed with acetonitrile (2 × 4 mL) and dried to get 0.65 g, (56%) of the product IIA. Yellow solid; Yield:
56%; IR (KBr, cm-1): 3046 (w), 2922 (w), 2360 (w), 1686 (s. ν(NO) belonging to IIIA), 1680 (s. ν(NO)), 1585
1 (w), 1379 (s), 1244 (m), 1211 (w), 1091 (w); H NMR (500 MHz, CDCl3): δ 0.52 (s, 3H), 0.62 (s, 3H), 2.19 (s,
31 3H), 6.65-7.85 (unresolved, 26 H); P NMR (121 MHz, CDCl3): δ -5.4 (br, s), -2.5 (br, s); Anal. (%). Calc for
C40H35Br2N2O2P2ReSi: C, 47.48 %; H, 3.49; N, 2.77 %. Found: C, 47.21; H, 3.37; N, 2.94.
Compound IIIA
-1 1 IR (KBr, cm ): 1686 (s. ν(NO); H NMR (300 MHz, CDCl3): δ 0.50 (s, 6H), 0.89 (s, 6H), 2.02 (s, 6H (lattice
CH3CN)), 6.65-6.80 (overlapping, 16H), 6.99 (t, 4h), 7.34-7.48 (overlapping 16H), 7.65-7.91 (overlapping,
31 16H) P NMR (121 MHz, CDCl3): δ 25.2 (s).
Re(B)(CH3CN)Br2NO (IIB) and Re(C)(CH3CN)Br2NO (IIC) were also prepared by this method, but with a different work up procedure for the latter. The reaction mass was filtered, washed with acetonitrile (2 x 2 mL). The combined acetonitrile layers were concentrated to dryness and extracted with THF (2 x 3 mL). This was concentrated to half of its volume and crystallized.
Compound IIB
Yellow solid; Yield: 52%; IR (KBr, cm-1): 3045 (w), 2923 (w), 2362 (w), 1686, (s. ν(NO)), 1570 (w), 1435 (m),
1 1377 (s), 1241 (m), 1211 (w), 1091 (w); H NMR (500 MHz, CDCl3): δ 2.25 (s, 3H), 6.76-7.85 (unresolved, 36
31 H; P{1H} NMR (121 MHz, CDCl3): δ -4.8 (br, s), -2.0 (br, s), 25.4 (s, belonging to IIIB); Anal. (%). Calc for
C50H39Br2N2O2P2ReSi: C, 52.87; H, 3.46; N, 2.47. Found: C, 52.55; H, 3.49; N, 2.21.
Compound IIC
Yellow solid; Yield: 47%; IR (KBr, cm-1): 3052 (w), 2955 (w), 2362 (w), 1702, (s. ν(NO)), 1617 (w), 1482 (m),
1 1435 (s), 1400 (s), 1221 (w), 1211 (w), 1096 (m); H NMR (300 MHz, CDCl3): δ 2.09 (s, br, 3H), 6.45-7.85
31 (unresolved, 26 H); P {1H} NMR (73 MHz, CDCl3): δ 14.7--13.68 (s, br), 26.1 (s, belonging to IIIC);
Anal.(%): Calc for C38H29Br2N2O2P2ReS: C, 46.30; H, 2.97; N, 2.84. Found: C, 45.99; H, 2.93; N, 2.65.
Preparation of [Re(oCPPh-A)(ɳ2-ethylene)Br(NO)] (VIIA) and [Re(oCPPh-B)(ɳ2-ethylene)Br(NO)] (VIIB)
Re(Sixantphos)(CH3CN)Br2NO (IIA) (1 g, 0.99 mmol) was taken in dichloromethane (8 mL) and added triethylsilane (3 mL, excess) followed by ethylene gas at 2 bar. The solution is allowed to stir at 70 oC for 6 h.
56 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
Cooled to room temperature and the solvent was evaporated to dryness. The obtained solids were washed with toluene (2 × 2 mL) and dried to get the product as a mixture of diastereomers IIA1 and IIA2 in a ratio of 1 : 0.4
( 0.634 g, 70%). The mixture is chromatographed on a silica gel column (Eluent: Hexane/Dichloromethane) to get the products IIA1 (0.373 g, 41%) and IIA2 (0.145 g, 16 %).
Compound VIIA1
Pale yellow solid; Yield: 41%; IR (KBr, cm-1): 3046 (w), 2922 (w), 2360 (w), 1680 (s), 1585 (w), 1379 (s), 1244
1 (m), 1211 (w), 1091 (w); H NMR (500 MHz, CDCl3): δ 0.44 (s, 3H), 0.59 (s, 3H), 1.76 (q, 1H, J = 9.0 Hz),
2.72 (m, 1H), 3.13 (q, 2H, J = 9 Hz), 6.78 (td, 2H, J = 7.0, 1.5 Hz), 7.05 (td, 3H, J = 7.0, 1.5 Hz), 7.13-7.20 (m,
5H), 7.24-7.34 (m, 6H), 7.37-7.43 (m, 3H), 7.53 (t, 1H, J = 7.5, 1.5 Hz), 7.62 (d, 1H, 7.5 Hz), 7.68-7.72 (m,
13 2H), 7.82 (t, 1H, J = 7.5 Hz), 8.08 (quin, 1H, 4.0 Hz); C {1H} NMR (125 MHz, CDCl3): -2.8, 0.7, 49.3 (d, J =
6.3 Hz), 49.7 (d, J = 9.3 Hz), 117.4 (d, J = 50.0 Hz), 117.2 (d, 40.1 Hz), 121.2 (d, J = 2.2 Hz), 121.3 ( d, J = 2.1
Hz), 123.6 (d, J = 6.3 Hz), 124.1 (d, J = 6.1 Hz), 124.3 (d, J = 6.1 Hz), 125.8 (d, J = 8.8 Hz), 127.9 (d, J = 3.2
Hz, 128.0 (d, J = 3.5 Hz), 128.7 (d, J = 10.5 Hz), 129.7 (d, J = 18.8 Hz), 129.8 (d, J = 2.9 Hz), 130.5 (d, J = 5.1
Hz), 131.7 (d, J = 38.7 Hz), 132.2 ( d, J = 10.6 Hz), 132.4 (dd, J = 35.9, 2.8 Hz), 132.8 (t, J = 3.0 Hz), 133.4
(dd, J = 44.5, 2.7 Hz), 134.6 (d, J = 11.2 Hz), 135.6 (d, J = 10.7 Hz), 137.1 (d, J = 12.5 Hz), 149.1 (dd, J = 48.1
Hz, 6.2 Hz), 152.6 (dd, J = 60.0, 6.9 Hz), 161.8 (d, J = 5.0 Hz), 162.0 (d, J = 5.2 Hz). 31P{1H} NMR (121 MHz,
CDCl3): δ -69.4 (d, J = 29.7 Hz), -12.7 (d, J = 29.7 Hz); Anal. (%). Calc for C40H35BrNO2P2ReSi: C, 52.34; H,
3.84; N, 1.53. Found: C, 52.68; H, 4.09; N, 1.53.
Compound VIIA2
Pale yellow solid; Yield: 16%; IR (KBr, cm-1): 3045 (w), 2923 (w), 2362 (w), 1686 (s), 1570 (w), 1435 (m),
1 1377 (s), 1241 (m), 1211 (w), 1091 (w); H NMR (500 MHz, CDCl3): δ 0.51 (s, 3H), 0.54 (s, 3H), 1.99 (q, 1H, J
= 8.0 Hz), 2.66 (m, 1H), 2.97-3.09 (m, 2H), 6.63 (td, 2H, J = 8.5, 2.5 Hz), 6.97 (tq, 1H, J = 8.5, 1.5 Hz), 7.12-
7.31 (m, 7H), 7.32-7.37 (m, 3H), 7.38-7.42 (td, 2H, J = 7.5, 2.5 Hz), 7.48-7.54 (m, 3H), 7.60-7.65 (m, 3H),
7.68-7.72, (m, 2H), 7.83 (td, 1H, J = 8.0, 1.5 Hz), 8.05 (quin, 1H, J = 4.0 Hz); 13C {1H} NMR (125 MHz,
CDCl3): -2.3, 0.6, 47.3 (d, J = 7.2 Hz), 48.4 (d, J = 10.0 Hz), 116.8 (d, J = 46.0 Hz), 117.2 (d, 37.4 Hz), 121.4
(d, J = 2.2 Hz), 121.5 ( d, J = 2.1 Hz), 124.1 (d, J = 5.6 Hz), 124.6 (d, J = 5.6 Hz), 125.9 (d, J = 8.5 Hz), 127.6
(d, J = 9.6 Hz, 127.9 (d, J = 9.0 Hz), 128.5 (d, J = 10.5 Hz), 129.2 (d, J = 38.8 Hz), 129.8 (d, J = 2.9 Hz), 129.9
(d, J = 2.9 Hz), 130.0 (d, J = 4.1 Hz), 130.4 ( d, J = 5.3 Hz), 130.8 (d, J = 18.5, 1.5 Hz), 132.7 (t, J = 3.0 Hz),
132.8 (dd, J = 41.2, 4.3 Hz), 133.3 (d, J = 9.3 Hz), 133.8 (d, J = 9.6 Hz), 134.1 (d, J = 2.8 Hz), 134.7 (d, J = 8.2
57 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Hz), 134.8 (d, J = 2.5 Hz), 135.6, 136.9, 137.4, 151.2 (dd, J = 40.1 Hz, 5.0 Hz), 153.1 (dd, J = 54.2, 7.9 Hz),
31 161.0 (d, J = 5.6 Hz), 161.9 (d, J = 6.8 Hz). P{1H} NMR (121 MHz, CDCl3): -76.2 (d, J = 34 Hz), -22.8 (d, J
= 29.3 Hz); Anal. (%). Calc for C40H35BrNO2P2ReSi: C, 52.34; H, 3.84; N, 1.53. Found: C, 52.71; H, 3.57; N,
1.51.
2 A similar procedure was adopted for the preparation of [Re(Sixantphos-Ph2)(η -ethylene)Br(NO)] (VIIB1 and
VIIB2)) from compound IIB. Anal. (%). Calc for C50H40BrNO2P2ReSi (1 : 0.3 mixture of diastereomers VIIB1 and VIIB2): C, 57.58; H, 3.87; N, 1.34. Found: C, 57.35; H, 4.01; N, 1.23.
Compound VIIB1
Pale gray solid; Yield: 47%; IR (KBr, cm-1): 3042 (w), 2989 (w), 2959 (w), 2916 (w), 2351(w), 1682 (s), 1573
(m), 1428 (m), 1408 (m), 1378 (s), 1261 (m), 1227 (m), 1206 (m), 1187 (m), 1103 (m), 1025 (m), 995 (m); 1H
NMR (500 MHz, CDCl3) δ 1.84 (q, 1H, J = 8.5 Hz, 2.81 (q, 1H, J = 8.5 Hz), 3.19 (q, 2H, J = 8.5 Hz), 6.75 (t,
2H, J = 7.5 Hz), 7.03-7.13 (m, 3H), 7.15 (t, 2H, J = 1.5 Hz), 7.13-7.23 (m, 3H), 7.27-7.36 (m, 8H), 7.40-7.44
(m, 4H), 7.52 (d, 4H, J = 7.5 Hz), 7.54-7.57 (m, 2H), 7.67 (m, 2H), 7.74 (d, 2H, J = 7.0 Hz,), 7.78 (t, 1H, J =
31 8.0 Hz) 7.89 (t, 1H, J = 8.0 Hz), 8.13 (quin, 1H, J = 4.0 Hz); P{1H} NMR (CDCl3, 121 MHz): δ -68.8 (d, J =
30.2 Hz), -11.7 (d, J = 30.2 Hz).
Compound VIIB2
Pale gray solid; Yield: 15%; IR (KBr, cm-1): 3045 (w), 2923 (w), 2362 (w), 1686 (s), 1570 (w), 1435 (m), 1377
1 (s), 1241 (m), 1211 (w), 1091 (w); H NMR (500 MHz, CDCl3): δ 2.07 (q, 1H, J = 8.5 Hz), 2.75 (m, 1H), 3.10
(m, 2H), (6.63 (t, 2H, J = 7.5 Hz), 6.98 (tq, 1H), 7.16-7.27 (m, 4H), 7.29-7.45 (m, 13H), 7.56 ( m, 6H), 7.67-
31 7.79 (m, 7 H), 7.90 (t, 1H, J = 8.0 Hz), 8.12 (quin, 1H, 4.0 Hz); P{1H} NMR (121 MHz, CDCl3): -75.5 (d, J =
30.2 Hz), -21.8 (d, J = 32.2 Hz).
Preparation of [Re(POP)(I)2(NO)] (XIIIA)
Complex IIIA (0.1 g, 0.099 mmol) and NaI (0.074 g, 0.494 mmol) was taken in a 5 mL Schlenk flask. Acetone
(1 mL) was added to it. The flask was closed and heated to 50 °C for 3 h. NaBr was observed to be precipitated out. The mixture was filtered, the solution was evaporated to dryness. It is extracted with dichloromethane (2 x
1 mL), evaporated and dried to get XIIIA as a yellowish brown solid (0.101 g, 0.0946 mmol, yield 96%). The same reaction was carried out in dichloromethane. The mixture after reaction was filtered, evaporated and dried to get XIIIA (0.1006 mg, yield 96%). IR (KBr, cm-1): 3052 (w), 2922 (w), 1688 (s), 1586 (m), 1482 (m), 1434
1 (s), 1392 (m), 1368 (s), 1254 (w), 1158 (w), 1096 (m); H NMR (500 MHz, CDCl3): δ 0.50 (s, 3H), 0.78 (s,
3H), 6.66-6.72 (m, 8H), 6.95 (t, J = 6 Hz, 2H), 7.40-7.43 (m, 8H), 7.69 (d, J = 7 Hz, 2H), 7.84-7.89 (m, 6H);
58 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
31 P{1H} NMR (121 MHz, CDCl3): δ 24.4; Anal. (%). Calc for C38H32I2NO2P2ReSi: C, 42.87; H, 3.03; N, 1.32.
Found: C, 43.04; H, 3.16; N, 1.34.
[Re(P)(CH3CN)2(Br)2NO] (P = D, XVID)
27 26 [Re(CH3CN)3(Br)2NO] (XV) (0.1 g, 0.2 mmol) and DBFmonophos (D) (0.074 g, 0.21 mmol) was taken in a glass autoclave and THF (1.5 mL) was was added to it. The vessel was closed and heated to 130 °C for 6 h. The yellow solids precipitated out were filrered. It is washed with THF (2 x 1 mL) and then with acetonitrile (1 mL). The solids were dried to get compound XIVD as a yellow solid (0.127 g, 0.156 mmol, yield 78%). IR
(KBr, cm-1): 3053 (w), 2970 (w), 2916 (w), 1703 (s), 1577 (m), 1482 (m), 1449 (s), 1435 (s), 1403 (s), 1367 (w),
1 1263 (w), 1187 (s), 1109 (w), 1093 (m), 1059 (m); H NMR (500 MHz, CDCl3): δ 2.42 (s, 3H), 2.46 (s, 3H),
7.27 (m, 1H); 7.33-7.43 (m, 10H), 7.76-7.82 (m, 4H), 7.97 (d, J = 7 Hz, 1H), 8.07 (d, J = 7 Hz, 1H), 31P{1H}
NMR (121 MHz, CDCl3): δ 1.7 (s); Anal. (%). Calc for C28H23Br2N3O2PRe: C, 41.49; H, 2.86; N, 5.18. Found:
C, 41.49; H, 3.01; N, 5.05.
Typical Procedure for the Hydrogenation of Olefins
Catalyst IIIA (0.005 mg, 0.00494 mmol), Styrene (15 mL, 130.92 mmol ) and Et3SiH (0.091 mL, 0.57 mmol) were taken in a 50 mL stainless steel autoclave. Toluene (10 mL) was added to it. The vessel was closed and connected to a Büchi pressflow gas controller machine. The gas line was evacuated thrice and the line was charged with approx. 3 bar of H2. The vessel was opened and it was evacuated carefully (thrice, not allowing pressure to go below 0 bar) to remove nitrogen. The vessel was charged with H2 (10 bar) and the mass was immediately kept in an oil bath maintained at appropriate temperature. The consumption of the gas is measured from the graph, from which the conversion of styrene could be calculated (Figure 2.7).
For the hydrogenation of dimethyl itaconate and phenyl acetylene, the reaction vessel was charged with
H2 (50 bar) and kept in an oil bath maintained at 140 °C. In the case of dimethyl itaconate, the vessel was cooled to room temperature after 1 h, 15 h, 23 h, 36 h (and samplings were done) and in all thse cases, again H2 (50 bar) was charged and kept in the oil bath. For both these substrates, the final reaction mass was analyzed by
GC/MS (CP-3800 Saturn 2000MS/MS spectrometer, column: Agilent VF-5ms, 30m x 0.25mm x 0.25µm) and the yields were calculated on the basis of consumption of the substrates.
Compound: retention time (mass pesk); Dimethyl itaconate: 3.44 min (m/z: 173), dimethyl methylsuccinate: 3.21 min (m/z: 175); Phenylacetylene: 2.06 min (m/z: 102), styrene: 2.15 min (m/z : 204),
59 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes ethylbenzene: 2.02 min (m/z: 106).
2.5. References
1. a) J. G. de Vries, C. J. Elsevier, in Handbook of Homogeneous Hydrogenation; Eds.; Wiley-VCH: Weinheim, 2007; 2. a) Zimmermann, S.; Sures, B. Environ. Sci. Pollut. Res. 2004, 11, 194-199; b) M. Schmid, S. Zimmermann, H. F. Krug, B. Sures, Environ. Int. 2007, 33, 385-390. 3. a) D. Heller, A. H. M. Vries, in Handbook of Homogeneous Hydrogenation, (Eds.: J. G. de Vries, C. J. Elsevier), Wiley-VCH: Weinheim, 2007; pp 1483-1516; b) Bartholomew, C. H. Appl. Catal. A 2001, 212, 17-60; c) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A Chem. 2003, 198, 317-341. 4. a) D. M. Heinekey, M. H. Voges, D. M. Barnhart, J. Am. Chem. Soc. 1996, 118, 10792–10802; b) C. Bianchini, A. Marchi, L. Marvelli, J. Am. Chem. Soc. 2011, 133, 8168-8178; c) M. Peruzzini, A. Romerosa, R. Rossi, A. Vacca, Organometallics 1995, 14, 3203-3215; c) D. Gusev, A. Llamazares, G. Artus, H. Jacobsen, H. Berke, Organometallics 1999, 18, 75-89. 5. a) A. Choualeb, O. Blacque, H. W. Schmalle, T. Fox, T. Hiltebrand, H. Berke, Eur. J. Inorg. Chem. 2007, 5246-5261; b) J. A. Gladysz, B. J. Boone, Angew. Chem., Int. Ed. Engl. 1997, 36, 550-583. 6. S. E. Clapham, A. Hadzovic, R. H. Morris, Coord. Chem. Rev. 2004, 248, 2201-2237. 7. A. Choualeb, E. Maccaroni, O. Blacque, H. W. Schmalle, H. Berke, Organometallics 2008, 27, 3474- 3481. 8. Y. Jiang, O. Blacque, T. Fox, C. M. Frech, H. Berke, Chem. Eur. J. 2009, 15, 2121-2128. 9. Y. Jiang, O. Blacque, T. Fox, C. M. Frech, H. Berke, Organometallics 2009, 28, 5493-5504. 10. a) Y. Jiang, J. Hess, T. Fox, H. Berke, J. Am. Chem. Soc. 2010, 132, 18233-18247; b) Y. Jiang, B. Schirmer, O. Blacque, T. Fox, S. Grimme, H. Berke, J. Am. Chem. Soc. 2013, 135, 4088-4102 11. a) R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 2134; b) R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 4450; c) R. H. Crabtree, A. Gautier, G. Giordano, and T. Khan, J. Organometal. Chem. 1977, 141, 113. 12. a) H. Berke, P. Burger, Comments Inorg.Chem. 1994, 16, 279-312; b) H. Jacobsen, H. Berke, in Recent Advances in Hydride Chemistry; (Ed.: R. Poli), Elsevier: Amsterdam, Holland, 2001; pp 89-116; c) A. Choualeb, E. Maccaroni, O. Blacque, H. W. Schmalle, H. Berke, Organometallics 2008, 27, 3474- 3481. 13. J. Chatt, S. Coffey, J. Chem. Soc. A 1969,1963-1969. 14. M. L. Clarke, J. J. R. Frew, Ligand electronic effects in homogeneous catalysis using transition metal complexes of phosphine ligands; Organometallic Chemistry, 2009, 35, 19-46. 15. P. W. N. M. van Leeuwen, Homogeneous Catalysis: Understanding the Art, Kluwer Academic Publishers, The Netherlands, 2004. 16. a) M. Kranenburg, Y. E. M. van der Burgt, P. C. J. Kamer, P. W. N. M. van Leeuwen Organometallics 1995, 14, 3081-3089; b) L. A. van der Veen, P. H. Keeven, G. C. Schoemaker, J. N. H. Reek, P. C. J. Kamer, P. W. N. M. van Leeuwen, M. Lutz, A. L. Spek, Organometallics 2000, 19, 872-883. 17. Gusev, D.; Llamazares, A.; Artus, G.; Jacobsen, H.; Berke, H.Organometallics 1999, 18, 75-89.
60 Chapter 2 Large Bite Angle Diphosphine Nitrosyl Rhenium Complexes as Highly Efficient Catalysts for Olefin Hydrogenations
18. B. Dudle, Thesis, 2010, University of Zurich, Switzerland. 19. von Zelewsky, A. Stereochemistry of coordi+nation compounds, In Inorganic Chemistry; Wiley: Chichester, U.K, 1995. 20. J. Chatt, L. A. Duncanson, J. Chem. Soc. 1953, 2939-2947. 21. C. P. Casey, G. T. Whiteker, Isr. J. Chem. 1990, 30, 299-304. 22. Crabtree, R. Acc. Chem. Res. 1979, 12, 331-337. 23. J. Chatt, S. Coffey, J. Chem. Soc. A 1969, 1963–1969. 24. a) A. Choualeb, O. Blacque, H. W. Schmalle, T. Fox, T. Hiltebrand, H. Berke, Eur. J. Inorg. Chem. 2007, 5246-5261; b) J. A. Gladysz, B. J. Boone, B, Angew. Chem., Int. Ed. Engl. 1997, 36, 550-583; c) A. Choualeb, E. Maccaroni, O. Blacque, H. W. Schmalle, H. Berke, Organometallics 2008, 27, 3474-3481. 25. a) J. A. Osborn, F. H. Jardine, J. F. Young, G. J. Wilkinson, Chem. Soc. A 1966, 12, 1711-1732; b) R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 2134-2143; c) C. R. Landis, J. Halpern, J. Am. Chem. Soc. 1987, 109, 1746-1754; d) R. H. Crabtree, A. Gautier, G. Giordano, T. Khan, J. Organomet. Chem. 1977, 141, 113-121. 26. M. W. Haenel, D. Jakubik, E. Rothenberger, G. Schroth, Chem. Ber. 1991, 124, 1705-1710; b) C. A. Wheaton , B. J. Ireland, P. G. Hayes, Organometallics 2009, 28, 1282-1285. 27. A. Choualeb, E. Maccaroni, O. Blacque, H. W. Schmalle, H. Berke, Organometallics 2008, 27, 3474- 3481.
61 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
(Major part of this work was published in K. Rajesh, B. Dudle, O. Blacque, H. Berke, Adv. Synth. Catal. 2011, 353, 1479-1484)
3.1. Introduction
There has been a strong interest in the hydrogenation of nitriles due to their facile access and frequent availability as commodity chemicals, the reaction has great potential in synthetic organic chemistry and in the production of pharmaceuticals, agrochemicals, textile and rubber chemicals.1 Apart from this aspect, syntheses of secondary amines are particularly significant in view of their role as biologically active molecules2,3 and versatile ligands.2,4
However, apart from the difficulty in their hydrogenation,1 a crucial selectivity problem arises leading to the formation of mixtures of primary, secondary and tertiary amine, as well as intermediate imines.1a,b,6 Although many proposals on the mechanism of the formation of secondary amines and tertiary amines have been put forward (Scheme 3.1 and Scheme 3.2), it is still unclear whether the reductions to amines occur through the hydrogenation of imines, enamines or the hydrogenolysis of gem-diamines.1a But what is mechanistically well
Scheme 3.1. Formation of different types of imines/amines in the hydrogenation of aromatic nitriles.
62 Chapter 9 Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
Scheme 3.2. Formation of different types of imines/amines/enamines in the hydrogenation of aliphatic nitriles
established, is the reaction between the formed primary amine and the intermediate imine
accompanied by the expulsion of ammonia, which can give rise to a symmetrical secondary
amine and repetition of this type of reaction sequence with the formed secondary amine. This
can lead to a symmetrical tertiary amine.1a,b,6 The composition of the reduction products depends mainly on the nature of the reducing agent, catalyst, reaction temperature and hydrogen pressure, and on the structure of the nitrile.2
Catalytic hydrogenation of nitriles using molecular hydrogen is of great interest as it is more efficient, economic and environmental friendly in processes as they have been applied in both academia and industry.1b,7 The formation of amines is often achieved by
heterogeneously8 or homogeneously catalyzed processes that involve the hydrogenation of
Nitriles. The homogeneous processes offer the opportunity for appropriate ligand sphere
turning there by imparting improved selectivity and operative at mild conditions as well as
understanding of their mechanisms. Homogeneous hydrogenations of nitriles are often
achieved using Ni,9 Ru,6a,10 Rh,5b Pd,11 Ir12 and Pt13 complexes. Alkylamines can also be
produced by amination of alcohols over acidic catalysts, however this tends to form also
undesired alkenes.14 Hydroaminomethylation of alkenes,15 reductive amination of carbonyl compounds, treatment of primary amines with alkyl halides or dialkyl sulfates or sulfonates,
63 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
addition of nucleophiles or radicals to N-substituted imines etc., have been widely reported
for the synthesis of secondary amines.3 However, the traditional methods for secondary amine formation are often problematic, because of harsh reaction conditions, generally poor yields and/or low chemoselectivities.3
3.2. Results and Discussion
3.2.1. Hydrogenation of Nitriles Catalyzed by Complexs of the Type II, III and VII
We tested the activity of complexes IIIA, IIB and VIIA-VIIB for the hydrogenation of
nitriles. All the four complexes were turned out to be catalysts and could hydrogenate nitriles
to the corresponding symmetrical secondary or tertiary amines in good selectivity. Although
the formation of N-benzylidenebenzylamine d was observed in the hydrogenation of
benzonitrile, no N-benzylideneamine b or benzylamine c was detected during the course of
the reaction. However, tribenzylamine f was observed as a side product. The formation of a
considerable amount of N-benzylidenebenzylamine during the course of the reaction and its
decrease at the end of the reaction indicated that N,N-dibenzylamine was formed by the
hydrogenation of N-benzylidenebenzylamine rather than by hydrogenolysis of the gem-
diamine formed by the reaction between b and c; whereas the formation of additional
tribenzylamine is attributed as a consequence of the hydrogenolysis of the gem-diamine. But
when the aliphatic nitrile, phenylacetonitrile was subjected to hydrogenation, diphenethyl-2-
phenylethenamine g’ was detected which revealed that the tertiary amine f’ was formed by
hydrogenation of the enamine g rather than the hydrogenolysis of gem-diamine.
Hydrogenation of benzonitrile using 0.1 mol% of VIIA was carried out under different
conditions and the selectivity of formation of various substituted amines are given in Table
3.1). An initial TOF of 90 h-1 was observed at 50 bar of hydrogen pressure and 140 °C in
THF as solvent (Table 3.1, entry 1). When 25 equivalents of triethylsilane with respect to the amount of the catalyst were added, the TOF was raised to 205 h-1 (Table 3.1, entry 3),
64 Chapter 9 Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
Table 3.1. Optimization table for the hydrogenation of benzonitrile[a]
[b] [c] Entry Et3SiH Pressure[bar]/ Selectivity [%] Initial Initial [equiv.] Catalyst after 2 h Conv.[c] TOF [h-1] d e f [%/2 h]
1 – 50/VIIA 28 72 – 90 18 2 – 50/ VIIA 7 80 13 – 90/16 h 3 25 50/ VIIA 17 79 4 205 41 4 25 50/ VIIA 4 66 30 – 90/18 h 5 5 50/ VIIA 8 86 6 161 32 6 50 50/ VIIA 11 85 4 208 42 7 25 30/ VIIA 7 82 11 135 27 [d] 8 25 50/ VIIA 33 62 5 79 16 [e] 9 25 50/ VIIA 7 62 31 156 31 10 25 50/IIIA 16 81 3 250 50 11 25 50/IIIA 8 73 19 – 91/18 h 12 25 50/IIB 17 81 2 229 46 13 25 50/VIIB 19 76 5 161 32 [a]0.1 mol% of catalyst was used, reaction at 140 °C in THF, TOFs were calculated as an average of the first 2 h. [b]With respect to catalyst. [c]By GC/MS. [d]Reaction at 120 °C. [e]Solvent: dichloromethane.
Table 3.2. Hydrogenation of phenylacetonitrile and cyclohexanecarbonitrile[a]
[b] [c] [c] Entry Et3SiH Pressure [bar]/ Selectivity [%] Initial TOF Initial Conv. [equiv.] Catalyst after 2 h [h-1] [%/2 h] D' e' f'
25 50/VIIA – 31 37 248 50[d] 1 25 50/ VIIA – 38 53 – 97[e] [f] 2 25 50/ VIIA 17 83 – 150 30 3 25 50/ VIIA 1 94 5 – 99[e] 4 [a]0.1 mol% of catalyst was used, reaction at 140 °C in THF, TOFs were calculated as an average of the first 2 h, entries 1-2 for phenylacetonitrile and 3-4 for cyclohexanecarbonitrile. [b]With respect to catalyst. [c]By GC/MS based on the consumption of the substrate, [d]32% of g' was formed. [e]Conversion in 18 h. [f]9% of g' was formed.
however, addition of 5 equivalents of triethylsilane was found to be inferior (Table 3.1, entry
5) and addition of 50 equivalents was found to have an effect comparable to the reaction with
25 equivalents of triethylsilane (Table 3.1, entry 6). A TOF of 156 h-1 was obtained, when
65 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
dichloromethane was used as the solvent, but gave 31% of tribenzylamine (Table 3.1, entry
9). When the reaction was carried out at a pressure of 30 bar of hydrogen, a TOF of 135 h-1
was accomplished with the formation of 11% of tribenzylamine (Table 3.1, entry 7), whereas
the reaction at 50 bar hydrogen and 120 °C gave a TOF of 79 h-1 with the formation of 5% of tribenzylamine (Table 3.1, entry 8). However, still longer reaction times using catalyst VIIA with 50 bar of hydrogen pressure at 140 °C and the addition of 25 equivalents of triethylsilane in THF gave 30% of tribenzylamine and 4% of N-benzylidenebenzylamine in
18 h (Table 3.1, entry 4), whereas without the addition of silane, only 13% of tribenzylamine and 7% of N-benzylidenebenzylamine were obtained within 16 h (Table 3.1, entry 2) with
90% conversions in both the cases. Although the initial TOF was comparatively higher, the longer reaction time would be accompanied by the formation of higher amounts of tertiary amine with much prominence in the former case there by reducing the selectivity. Under the former conditions of addition of triethylsilane, reaction with catalysts IIIA, IIB and VIIB
gave initial TOFs of 250 h-1, 229 h-1 and 161 h-1, respectively (Table 3.1, entries 10, 12 and
13).
The hydrogenation of the aliphatic phenylacetonitrile using catalyst VIIA under these
conditions gave a TOF of 248 h-1, with a conversion of 50%, but showed selectivity towards
triphenethylamine f’ (Table 3.2, entry 1) and continuing this reaction gave a conversion of
97% with selectivities of 53% of triphenethylamine f’, 38% of diphenethylamine e’ and 9%
of diphenethyl-2-phenylethenamine g’ in 18 h (Table 3.2, entry 2). The selectivity toward the
tertiary amine in this case can be due to higher reactivity towards nucleophilic attack on the
non-conjugated aliphatic imine functionality. Hydrogenation of the aliphatic
cyclohexanecarbonitrile gave a TOF of 150 h-1 with a conversion of 30%, but interestingly it
showed selectivity towards the secondary amine rather than the tertiary amine (Table 3.2,
entry 3), which can be attributed as a consequence of the bulkiness, as well as electron
66 Chapter 9 Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
richness, which is expected to retard the nucleophilic attack on this system. Further runs of this reaction gave a conversion of 99% in 18 h with selectivities of 94% of bis(cyclohexylmethyl) amine e’ along with 5% of tris(cyclohexylmethyl) amine f’ and 1% of cyclohexylidene(cyclohexylmethyl) methanamine D’ (Table 3.2, entry 4).
Keeping the other parameters unchanged, the selectivity of the hydrogenation of benzonitrile was reversed (Table 3.1, entries 2 and 4 in comparison with Table 3.3 entries 1 and 2) with respect to silane with a catalyst loading of 0.5 mol% at a pressure of 75 bar
(optimized conditions) gave a TOF of 198 h-1 with a conversion of 99% with selectivities of
90% of N,N-dibenzylamine, 4% of tribenzylamine and 6% of the N-benzylidenebenzylamine
(Table 3.3, entry 2) in 1 h. However, only very little improvement in hydrogenation was
Table 3.3. Hydrogenation of nitriles under optimized conditions[a]
Entry Nitrile Cat. Selectivity [%][c] TOF Conv.
Ar[b] d e f (h-1) (%)[c]
1 Ph VIIA [d] 10 66 24 180 90 2 Ph VIIA 6 90 4 198 99 3 Ph IIIA 5 85 10 198 99 4 Ph IIB 8 86 6 198 99 5 Ph VIIB 7 82 11 196 99 6 3-Tolyl IIIA 2 89 9 198 99 7 3-Tolyl IIB 4 87 9 198 99 8 2-Thienyl VIIA 5 83 12 162 99 9 2-Thienyl VIIIB 5 85 10 160 99 10 3-Tolyl VIIA 2 81 17 170 99 11 3-Tolyl VIIB 2 80 18 169 99 [e] 12 PhCH2 IIIA - 10 85 176 99 13 Cyclohexyl IIIA 2 95 3 198 99 [a] 0.5 mol% of catalyst with 25 equiv. of triethylsilane with respect to catalyst at 75 bar H2, 140 °C in THF, run for 1 h except for entries 8-12 for which reactions were run for 4 h and the given TOFs are for the first hour. [b]Ar = R for entries 12 and 13 which corresponds to scheme 3.2, wherein d = D', e = e', f [c] = f' and for entry 13, RCH2 = cyclohexyl. By GC/MS based on the consumption of the substrate and for entry 2, a quantification using naphthalene as an internal standard was also adopted. [d]No silane was added. [e]5% of g' was formed.
67 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
observed even after 3 h, but an increased portion of tertiary amine (6%) and a reduced
amount of N-benzylidenebenzylamine (4%) was noticed. Under these conditions, reaction in
the absence of silane gave a TOF of 180 h-1 with 90% conversion forming 66% of N,N- dibenzylamine, 24% of tribenzylamine and 10% of N-benzylidenebenzylamine in the first 1 h
(Table 3.3, entry 1). The generality of the reaction was then tested by applying all the other catalysts in the benzonitrile reduction and reductions of a few other nitriles under the given optimized condition (Table 3.3).
To exclude any heterogeneous reaction course in the hydrogenation catalyses,
filtration and mercury poisoning experiments were carried out.[20] The hydrogenation of
benzonitrile was carried out using catalyst VIIA under the conditions of Table 3.3. The
reaction was stopped at 20 min and the obtained clear pale yellow solution, which showed a
conversion of 71%, was filtered through a plug of celite into a new vessel with a new stirring
bar, and the reaction was continued for another 40 min under the same conditions showed
identical results to Table 3.3, entry 2. Mercury poisoning experiment was carried out on the
hydrogenation of benzonitrile using catalyst VIIA under the conditions of Table 3.3. In the
presence of 60 equiv. of Hg (per Re atom) the reaction again showed identical results to
Table 3, entry 2. The filtration and mercury poisoning tests were thus all negative which rule
out any colloid or amalgam formation. At this point, it is also worth mentioning that rhenium
metal has a very high atom binding energy (second largest in the Periodic Table of Elements)
possessing therefore low propensity for colloid or amalgam formation.
3.2.2. Mechanistic Studies
As a step to elucidate the mechanism of this transformation, in a Young NMR tube complex
IIIA was reacted with 2 bar H2 pressure at 100 °C in dichloromethane for 2 h. Analysis of
this sample showed mixture of products according to 31P NMR spectroscopy. However,
leaving the sample for a week led to the formation of single crystals suitable for X-ray
68 Chapter 9 Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
diffraction studies. This was thus analyzed to be the complex XVIA where the CH3CN ligand in IIA (from which the dinuclear complex IIIA was formed; Chapter 2) was replaced by a
NH3 ligand (Figure 3.1). The formation of NH3 further provided evidence for the ability of this complex to hydrogenate nitriles to secondary or tertiary amines. Acetonitrile was hydrogenated completely to ethylamine, which reacted with the intermediate acetylimine to form imines or amines of a higher degree of substitution expelling thereby ammonia, which then coordinated to the rhenium center. Since stoichiometric quantities of ammonia could not
NO P Br O Re P NH3 Br XVIA
Figure 3.1. Complex XVIA and its molecular structure. Anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. Selcected bond lengths: Re1-Br1a: 2.487(1), Re1-Br2: 2.6065(7), Re1-N2: 2.200(4). Selected bond angles: P1ReP2: 96.10(4).
NO P Br O Re P N Ph Br H
XVIIA
Figure 3.2. Complex XVIIA and its molecular structure. Anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond angles: P1ReP2: 96.19(7). Selected bond lengths: Re1-Br1: 2.598(1), Re1-Br2: 2.5785(9), Re1-N2: 2.139(6).
69 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
have formed with respect to IIIA, we expected that imines and amines could be found to be coordinated to the rhenium centre similar to ammonia in XVIA.
Complex IIIA was then reacted in a pressure tube with 2 equiv. of benzonitrile under 2 bar
H2 pressure at 140 °C in THF for 2 h. When cooled to room temperature and left for a week,
tiny crystals were seen. This was subjected to an X-ray diffraction study showing a molecular
structure of XVIIA bearing a N-benzylideneamine ligand formed through partial
hydrogenation of benzonitrile (Figure 3.2).
These experiments showed the ability of complexes IIIA and IIB to hydrogenate nitriles without any co-catalyst. The obtained structures from the stoichiometric reactions suggest that a mechanism, which often require only one vacant site on the metal centre16 or
the dissociation of either a bromide ligand or a phosphine moiety would be operative.
However, an ionic mechanism for the splitting of H2 seems less probable could be ruled out
since these complexes were found to be active also in the hydrogenation of olefins and the
hydrogenation of olefins is not expected to be operative through heterolytic splitting of H2.
Further details will be explored in chapter 4.
The reactions using IIIA or IIB as a catalyst in the presence of Et3SiH, as well he
reactions using VIIA or VIIB as a catalyst in the presence or absence of Et3SiH, are
presumably be operating through a catalytic cycle as described for the hydrogenation of
olefins (Chapter 2) with complex of the type V as the active species, but starting from an ɳ1-
coordinated nitrile to the rhenium centre.
It is worth mentioning at this stage that in the absence of H2, i.e. the reaction between
Et3SiH and aromatic nitriles in the presence of IIIA or VIIA revealed at 80 °C mono
hydrosilylation products (Chapter 9). This reaction was far less efficient for the aliphatic
nitriles. However, we could not detect any hydrosilylated products under hydrogenation
70 Chapter 9 Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
conditions discussed in this chapter. Thus, an alternative pathway for the formation of higher
substituted imines through the activated N-silylimines is depicted in Scheme 3.3.
Scheme 3.3. Influence of hydrosilylation in the hydrogenation of aromatic nitriles.
3.3. Conclusion
In conclusion, we have developed an efficient air stable homogeneously rhenium-
catalyzed hydrogenation of nitriles with good selectivities for symmetrical secondary
amines or tertiary amines. Addition of triethylsilane could increase the TOFs and
suppress overalkylation of the amines at higher pressure with a relatively high loading of
the catalyst. Secondary amines are anticipated to be formed by the hydrogenation of the
imine (for aryl nitriles) or enamine (for alkyl nitriles) intermediates generated by the
elimination of ammonia from the gem-diamine species whereas the tertiary amines were
formed by the hydrogenolysis of the gem-diamines (for aryl nitriles) or hydrogenation of
the enamines (for alkyl nitriles). Rhenium as a neighbouring element to precious metals
can be ligand sphere tuned to adopt similar catalytic properties providing an appropriate
alternative to precious metal catalyses.
3.4. Experimental Section
3.4.1. General procedure for the catalytic hydrogenation of benzonitrile
Catalyst VIIA (0.002 g, 2.18 ×10-3 mmol) was taken in a stainless steel autoclave and was added benzonitrile
(0.045 g, 0.44 mmol) followed by THF (0.2 mL) and triethyl silane (9 µL, 5.63 × 10-2) Naphthalene (0.028 g,
0.22 mmol) was added as internal standard. It was pressurized with 75 bar of hydrogen and kept in an oil bath
71 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes maintained at 140 °C. After appropriate reaction time, the vessel was immediately cooled to room temperature and the hydrogen was released slowly in a fume hood. The reaction mixture was filtered through a short plug of silica gel and the ratio of product and conversion were measured by GC/MS (CP-3800 Saturn 2000MS/MS spectrometer, column: Agilent VF-5ms, 30m x 0.25mm x 0.25µm).
GC/MS data for nitriles, imines and amines
3.5. References
1. a) S. Gomez, J. A. Peters, T. Maschmeyer, Adv. Synth. Catal. 2002, 344, 1037-1057; b) B. Chen, U. Dingerdissen, J. G. E. Krauter, H. G. J. L. Rotgerink, K . Mobus, D. J. Ostgard, P. Panster, T. H. Riermeier, S. Seebald, T. Tacke, H. Trauthwein, Appl. Catal. A: Gen. 2005, 280, 17-46; 2. A. Galan, J. de Mendoza, P. Prados, J. Rojo A. M. Echavarren, J. Org. Chem. 1991, 56, 452-454. 3. a) R. N. Salvatore, C. H. Yoon, K. W. Jung, Tetrahedron 2001, 57, 7785-7811; and see reference therein; b) M. Freifelder, Catalytic Hydrogenation in Organic Synthesis: Procedures and Commentary; (Wiley: New York, 1978; Chapter 10). 4. a) R. A. Grey, G. P. Pez, A. Wallo, J. Am. Chem. Soc. 1981, 103, 7536-7542; b) T. Yoshida, T. Okano, S. Otsuka, J. Chem. Soc., Chem. Commun. 1979, 870-871 5. a) B. Miriyala, S. Bhattacharyya, J. S. Williamson, Tetrahedron, 2004, 60, 1463-1471; b) A. Togni, L. M. Venanzi, Angew. Chem. Int. Ed. 1994, 33, 497-526; c) M. Sawamura, Y. Ito, Chem. Rev. 1982, 92, 857- 871. 6. R. Reguillo, M. Grellier, N. Vautravers, L. Vendier, S. Sabo-Etienne, J. Am. Chem. Soc. 2010, 132, 7854- 7855; d) J. von Braun, G. Blessing, F. Zobel, Ber. Dtsch. Chem. Ges. 1923, 56, 1988-2001; e) G. Mignonac, Comptes Rendus 1920, 171, 14. 7. H.-U. Blaser, M. Studer, Appl. Catal. A: Gen. 1999, 189, 191-204. 8. a) L. Hegedus, T. Mathe, T. Karpati, Appl. Catal. A: Gen. 2008, 349, 40-45; b) L. Hegedus, T. Mathe, Appl. Catal. A, 2005, 296, 209-215; c) P. Kukula, M. Studer, H.-U. Blaser, Adv. Synth. Catal. 2004, 346, 1487-1493.
72 Chapter 9 Homogeneous Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
9. a) P. Zerecero-Silva, I. J.-Solar, M. G. Crestani, A. Arevalo, R. Barrios-Francisco, J. J. Garcia, Appl. Catal. A: Gen. 2009, 363, 230-234; b) B.W. Hoffer, J. A. Moulijn, Appl. Catal. A: Gen. 2009, 352, 193- 201. 10. a) D. Addis, S. Enthaler, K. Junge, B. Wendt, M. Beller, Tetrahedron Lett. 2009, 50, 3654-3656; b) S. Enthaler, D. Addis, K. Junge, G. Erre, M. Beller, Chem. Eur. J. 2008, 14, 9491-9494. c) D. K. Mukherjee, B. K. Palit, C. R. Saha, J. Mol. Catal. 1994, 88, 57-70. 11. A. Bose, C. R. Saha, J. Mol. Catal.1989, 49, 271-283. 12. C. S. Chin, B. Lee, Catal. Lett. 1992, 14, 135-140. 13. F.R. Hartley, The Chemistry of Platinum and Palladium, (Applied Science Publishers, London, 1973). 14. K. S. Hayes, Appl. Catal. A: Gen. 2001, 221, 187-195. 15. A. Seayad, M. Ahmed, H. Klein, R. Jackstell, T. Gross, M. Beller, Science 2002, 297, 1676-1678. 16. R. M. Bullock, in The Handbook of Homogeneous Hydrogenation (ed.: J. G. de Vries and C. J. Elsevier), Wiley-VCH Verlag GmbH, Weinheim, Germany, 2008, Ch. 7.
73 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
4.1. Introduction
When aldehydes or ketones are reacted with ammonia or primary amines, carbinolamines,
imines or enamines may be formed and when they are reacted with secondary amines,
carbinolamines or enamines are obtained, which are subsequently reduced to primary,
secondary and tertiary amines, respectively. Such processes are called reductive amination
with respect to the carbonyl compound or reductive alkylation with respect to the amine. The
reaction is a versatile tool in synthetic organic chemistry for the preparation of various
amines, other synthetic intermediates, pharmaceuticals, agrochemicals and rubber chemicals.1
Several borohydrides,2a-t silyl hydrides in combination with metal catalysts3 or
organocatalysts4 or acids,5 metal hydrides,6 metal acids7 and transition metal complexes with formate salts,8 Hantzsch dihydropyridines,9 as well as benzothiazoline10 etc, have been reported as reducing agents for the reductive amination of aldehydes and ketones. However, most of these reagents have one or more drawbacks, like the use of stoichiometric amounts of the reagents, toxicity of these, side reactions, harsh reaction conditions, the requirement of application of excess amine or ammonia and difficult work-up procedures.[2b,11] Treatment of primary amines with alcohols, alkyl halides, dialkyl sulfates or sulfonates comprise alternative N-alkylations, addition of nucleophiles or radicals to N-substituted imines were often reported as alternative syntheses of secondary amines, but all these methods still hold
74 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
Scheme 4.1. General scheme for the formation of various amines/imines/enamines in the reductive amination of aldehydes or ketones.
any of the above mentioned drawbacks.11 Catalytic reductive aminations using molecular hydrogen are of great interest as they are more efficient, economic and environmentally friendly in their conversions and are suitable to both academic laboratories and industry.1,13a-c
Preliminary studies began in 1974 when Mark′o and Bakos reported the first example using
13d molecular H2 applying Co and Rh carbonyl complexes as catalysts. Further then, especially in the last decade many reports arose including asymmetric versions, which apply homogeneous transition metal based hydrogenations.13 However, many of these methods require harsh reaction conditions and the catalytic treatments suffer from low selectivity, because of low tolerance to functionalities. For instance, groups like –NO2 or electron poor heterocycles were not seem to react or survive, or were not studied under these reaction conditions.
A typical problem facing the reductive amination is the reduction of the carbonyl group before it condenses with the amine or ammonia.1a-b,5,13b Generation of over-alkylation products may also be a major issue in this reaction (Scheme 4.1).
75 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
4.2. Results and Discussion
4.2.1. Reductive Amination of Aldehydes Using Complexes of the Type II, III and VII
Following the development of rhenium(I) complexes, which showed remarkable activities in hydrogenations of olefins (Chapter 2) and nitriles (Chapter 3) we studied their activity towards direct reductive aminations of aldehydes, as well as the hydrogenation of imines using molecular hydrogen. The reductive amination of aldehydes utilizes equimolar amounts of aldehyde and amine in THF, which can give up to 97% yield of the desired secondary amines during hydrogenation with relatively low catalyst loadings. To study a representative example, stoichiometric amounts of benzaldehyde (1a) and aniline (2a) were subjected to reductive amination with 0.005 mol% of the catalyst IIIA under a pressure of 30 bar H2 and at a temperature of 120 °C in THF. N-benzylaniline (3aa) was obtained with a TOF of 2177 h-1 without formation of benzyl alcohol, but 1% of N,N-dibenzylaniline appeared with 14% conversion during the first hour. The reaction showed a conversion of 97% in 18 h with formation of 16% of N-benzylaniline (3aa), 75% of the tertiary N,N-dibenzylaniline (7aa)
and 6% of the gem-diamine 6aa (Table 4.4.1, entry 1). In order to improve the selectivity for
the desired secondary amine 3aa, the reaction was carried out with 0.1 mol% catalyst loading
under a pressure of 50 bar and at 90 °C; a TOF of 559 h-1 in the first hour was noticed, which
gave 95% yield of the desired product 3aa in 3 h with 3% of benzyl alcohol (Table 4.4.1,
entry 2). It is worth mentioning that similar results were obtained when a mixture of IIA and
IIIA (~ 2:1 mixture obtained directly after preparation) was used as the catalyst instead of
only IIIA. Apart from the desired product, the major reaction component during sampling
was found to be the corresponding imine 5aa (>95%). Thus, it became evident that N-
benzylaniline (3aa) was formed via the intermediacy of the imine 5aa, which became
hydrogenated, rather than via hydrogenolysis of the carbinolamine 4aa. However, the
hydrogenolysis of 4 to form 3 is expected to be the route when aldehydes, which do not
76 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
possess α-hydrogen atoms, are subjected to this reaction with secondary amines. The formation of N,N- dibenzylaniline (7aa) can be envisaged as a consequence of the hydrogenolysis reaction of the gem-diamine 6aa formed by the reaction between the imine
5aa and N-benzylaniline (3aa), rather than by the reaction between 3aa and benzaldehyde followed then by hydrogenolysis, which was indicated by generation of only < 3% of benzaldehyde (Scheme 4.1). On attempts to understand the efficiency of the catalyst toward the production of the desired product 3aa, the catalyst loading was adjusted to 0.05 mol%
-1 under 50 bar of H2 pressure at 90 °C (optimized condition), which showed a TOF of 762 h in the first hour with the same results as with 0.1 mol%, but in 6 h (Table 4.4.1, entry 3).
Further attempts to optimize the selectivity of the reaction did not lead to better results.
Table 4.4.1. Reductive amination of benzaldehyde with aniline.[a]
Entry Cat./mol% P(bar)/ Time TOF (h-1) Yield (%) Yield (%) Yield (%) Conv. T(°C) (h) (1st h) (3ab) (6aa/7aa) (11a) (%) 1 IIIA/0.005 30/120 18 2177 16 6/75 - 97 2 III/0.1 50/90 3 559 95 -/- 3 99 3 IIIA/0.05 50/90 6 762 95 -/- 3 99 4 IIIA/0.05 30/120 3 1095 66 10/13 6 95 5 IIIA/0.05 50/120 2 1519 81 4/7 5 97 6 IIIA/0.1 50/120 <1 >960 96 -/- 3 99 7 IIB/0.05 50/90 7 746 96 -/- 3 99 8 VIIA/0.05 50/90 6 557 95 -/- 3 99 9 VIIB/0.05 50/90 7 541 95 -/- 3 99 10 IIIA/0.05 50/90 4 1066 96 -/- 3 99[b] [a]Yield, conversion and selectivity by GC/MS based on the consumption of aldehydes or aniline; TOFs for the formation of tertiary amines are excluded; reactions were carried out in THF. [b]100 equivalents of
n-Bu4NBr with respect to catalyst was added.
77 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Applying the same loading of the catalyst, under 30 bar H2 at 120 °C the reaction showed a
TOF of 1095 h-1 with formation of 1% of benzyl alcohol and 2% of the tertiary amine 7aa in
55% conversion in the first hour, but gave only 66% yield of the desired product 3aa, 13%
yield of the tertiary amine 7aa, 10% yield of the gem-diamine 6aa along with 6% of benzyl
alcohol in 95% conversion in 3 h (Table 4.4.1, entry 4). Continuation of this reaction for
another 3 h showed an increase in the tertiary amine 7aa to 30% and decrease in both the
gem-diamine 6aa and the desired product 3aa to 5% and 48% respectively, with a conversion
of 97%. This further confirmed that the formation of the tertiary amine, N,N-dibenzylamine
(7aa) proceeded via the gem-diamine 6aa. Another run of the reaction with a pressure of 50
-1 bar H2 at 120 °C showed an initial TOF of 1519 h , but gave only 81% of the desired product
3aa along with 7% of the tertiary amine 7aa, 4% of the gem-diamine 6aa and 5% of benzyl
alcohol with 97% conversion in 2 h (Table 4.4.1, entry 5). At this temperature, a still higher
loading of the catalyst (0.1 mol%) furnished 96% of the desired product 3aa along with 3%
of benzyl alcohol with almost complete conversion (Table 4.4.1, entry 6). Under optimized
conditions catalyst IIB bearing phenyl groups on the silicon atom of the diphosphine ligand
furnished a TOF of 746 h-1 in the first hour giving rise to the desired N-benzylaniline (3aa) in
96% yield along with 3% of the benzyl alcohol as a side-product and 99% total conversion
(Table 4.4.1, entry 7). The related ortho metallated rhenacyclic complexes VIIA and VIIB
showed TOFs of 557 h-1 (Table 4.4.1, entry 8) and 541 h-1 (Table 4.4.1, entry 9), respectively,
in the first hour, also with a conversion of 99%, along with 3% of benzyl alcohol and a yield
of 95% to the desired product 3aa in both the cases.
The generality of this reductive amination reaction was then tested with a variety of
aldehydes and amines in the presence of catalyst IIIA (Table 4.4.2). Reductive amination of
benzaldehyde with the electron rich 4-iodoaniline showed a TOF of 1434 h-1 in the first hour
with 62% yield of the desired product 3ab, but underwent deiodination of both the imine and
78 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
the product when run for the extended period of time of 15 h giving rise to 2% of the
corresponding iodide-free imine, N-benzylideneaniline (5aa), 90% of the deiodinated secondary amine N-benzylaniline (3aa) and 4% of the deiodinated tertiary amine, N-N- dibenzylaniline (7aa), over all with a conversion of 96% (Table 4.4.2, entry 1). The electron rich 4-anisaldehyde furnished upon reductive anilation a TOF of 1107 h-1 in the first hour with 96% yield of the desired amine N-(4-methoxybenzyl)aniline (3ba) in 3 h along with 3% of 4-methoxybenzyl alcohol (Table 4.4.2, entry 2). The electron deficient 4-nitroaniline upon reductive alkylation with benzaldehyde showed an initial TOF of 376 h-1 giving rise to 92% yield of the desired product, N-benzyl-4-nitroaniline (3ac) along with 4% of benzyl alcohol
(11a) in 9 h (Table 4.2, entry 3). At a higher loading of 0.3 mol% of catalyst IIIA at 120 °C the reductive anilation of the electron deficient aldehyde, 3-nitrobenzaldehyde provided an initial TOF of 171 h-1 giving rise to the desired amine, N-(3-nitrobenzyl)aniline (3ca) in 94% yield along with 3% of 3-nitrobenzyl alcohol in 15 h (Table 4.2, entry 4). Thus, it became evident that imines bearing an electron deficient aldehydic part were much more difficult to hydrogenate when compared to substrates with an electron deficient amine part. Under optimized conditions, reductive anilation of 4-chlorobenzaldehyde did not provide complete conversion even at the higher temperature of 120 °C run for 24 h so that a higher catalyst loading of 0.15 mol% was considered in combination with a temperature of 90 °C. This gave an initial TOF of 135 h-1 with 94% yield of the desired amine, N-(4-chlorobenzyl)aniline
(3da) and 4% of 4-chlorobenzyl alcohol in 15 h (Table 4.2, entry 5). A higher loading of 0.2 mol% of IIIA showed in the case of 1-napthaldehyde a TOF of 84 h-1 and a yield of 91% of the desired amine, N-(1-naphthylmethyl)aniline (3ea) and 4% of 1-naphthalene methanol in
15 h (Table 4.2, entry 6). By reductive anilation the heterocyclic 2-thienylcarboxaldehyde was converted smoothly into the desired amine under optimized conditions with an initial
79 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Table 4.2. Reductive amination of various aldehydes with different amines using complex IIIAa
IIIA Time TOF Yield Conv. Entry Aldehyde (1) Amine (2) (mol%) (h) (1st h) 3 (%)[b] (%)
1 1434 3ab 62 63
0.05 1 1a 2b 15 - 3aa - 96c
2 1b 2a 0.05 3 1107 3ba 96 99
3 1a 2c 0.05 9 376 3ac 92 97
NH 2
4 1c 2a 0.30 15 171 3ca 94 97d
5 1d 2a 0.15 15 135 3da 94 98
6 1e 2a 0.20 15 84 3ea 91 95
7 1f 2a 0.05 4 871 3fa 97 99
8 1g 2a 0.10 1 338 3ga 34 91d,e
9 1a 2d 0.10 15 178 3ad 94 96d
80 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
10 1h 2d 0.20 15 37 3hd 76 99d,f
11 1a 2e 0.10 15 - 3ae - 92d,g
O 12 76d,h,i 1i H H 2a 0.05 10 - 3ia 68
O 13 1i 2f 0.20 15 - 3if 77 77i H H
14 1a 2g 0.05 1 620 3ag 31 97
15 1a 2f 0.05 8 224 3af 22 97
j 16 1j 2a 0.05 15 164 3ja 66 96
a Unless and otherwise mentioned, all reactions were carried out under 50 bar of H2 at 90°C in THF; yield and conversion by GC/MS based on the consumption of amine or aldehyde. bUnless otherwise mentioned, remaining is the corresponding alcohol. cAll deiodination products; 90% N-benzylaniline (5aa) along with 4% of tertiary amine N,N-dibenzylaniline (7aa), remaining being the imine 5aa. dReaction was carried out at 120 °C. eOther products were higher amines, imines, enamines etc. f6% of isomeric imine 9 and 15% of 10 were formed. gReaction was carried out in 7 N methanolic ammonia, 43% of benzylidenebenzylamine was formed. h8% of N,N-dimethylaniline was formed. iParaformaldehyde was used. j8% of N-(3- phenylpropyl)aniline and 10% of 3-phenylpropanal were formed.
TOF of 871 h-1 and a yield of 97% of the desired amine, N-(2-thienylmethyl)aniline (3fa) along with 2% of the corresponding alcohol in 4 h (Table 4.2, entry 7). The aliphatic isobutyraldehyde was reductively anilated with a catalyst loading of 0.1 mol% at the higher temperature of 120 °C accomplishing a TOF of 338 h-1 and a yield of 34% of the desired amine, N-isobutylaniline (3ga) in one hour (Table 4.2, entry 8). Reductive amination of benzaldehyde with the aliphatic 1-hexylamine proceeded smoothly with a catalyst loading of
0.1 mol% at atemperature of 120 °C revealing an initial TOF of 178 h-1 and a yield of 94% of
81 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
the desired amine, N-benzyl-1-hexylamine 3ad along with 2% of benzyl alcohol in 10 h
(Table 4.2, entry 9). Reductive amination of the aliphatic aldehyde, hexanal with aliphatic amine, 1-hexylamine showed a TOF of 37 h-1 in the first hour giving rise to a yield of 76% of
the desired secondary amine, N,N-dihexyl-1-amine (3hd) in 15 h with 99% conversion, when a catalyst loading of 0.5 mol% was chosen at 120 °C (Table 4.2, entry 10). 6% of the isomeric imine 9hd was also formed at the end of the reaction which was observed only in
traces when sampling was done in the first hour. Reductive amination with an ammonia
solution in methanol (7 N, 8 equiv.) showed no yield of the desired benzylamine in 15 h,
instead 43% of the benzylamine formed was transformed further with benzaldehyde or
benzylideneamine to yield benzylidenebenzylamine apart from 6% benzyl alcohol (Table 4.2,
entry 11). An excess of paraformaldehyde (2 equiv.) was used at 120 °C for the reductive
methylation of aniline. A yield of 68% of N-methylaniline (3ia) and 8% of N,N-
dimethylaniline was obtained, all with respect to aniline, 24% of aniline remained due to the
concomitant hydrogenation of formaldehyde to methanol (Table 4.2, entry 12). Under the
same conditions, a still higher loading (3 equiv.) of paraformaldehyde with 0.2 mol% of the
catalyst was adopted accomplishing reductive methylation of diphenylamine to give 77%
yield of the desired tertiary amine, N,N-diphenylmethylamine (3if) in 15 h, all with respect to
diphenylamine (Table 4.2, entry 13). Reductive amination of equimolar benzaldehyde with
the secondary amines piperidine and diphenylamine gave only 31% and 22% yield,
respectively, of the desired tertiary amines, N,N-diphenylbenzylamine (3ag) and N-
benzylpiperidine (3af), due to prevailing hydrogenation of benzaldehyde to benzyl alcohol
(Table 4.2, entries 14, 15). α,β-unsaturated trans-cinnamaldehyde showed upon reductive
anilation specific formation of the desired enamine, N-(3-Phenyl-2-propenyl)benzenamine
(3ja) in the first hour with 16% conversion, but showed reduced selectivity with 66% yield of
82 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
the product 3ja along with 8% of N-(3-phenylpropyl)benzenamine and 10% of 3-
phenylpropanal when run for 15 h (Table 4.2, entry 16).
4.2.2. Hydrogenation of Imines Using Complexes of the Type IIIA and XIIIA
Following the reductive amination of aldehydes which led to the intermediacy of imines, we
tested IIIA for the hydrogenation of imines including few involved in the reductive
amination process discussed above (Table 4.3). Hydrogenation of N-benzylideneaniline 5aa, was carried out with a loading of 0.05 mol% of IIA and under 50 bar H2 pressure at 90 °C in
THF to reveal 99% yield of the desired amine 3aa within one hour. Consequently, the
monitoring samples of this imine hydrogenations were taken in the first 0.25 h. Under the
same conditions, but at a relatively lower loading of only 0.02 mol% of complex IIIA
showed a TOF of 3910 h-1 in the first 0.25 h giving rise to 97% yield of the desired product
3aa in < 3 h (Table 4.3, entry 1). The comparatively low activity in the reductive amination
when compared to the imine hydrogenation would be due to several reasons. Apart from the
concomitant hydrogenation of the aldehydes to the corresponding alcohols, reversible or
irreversible coordination of the aldehydes, as well as its hydrogenated product, alcohols to the
catalyst can slow down the catalytic cycle by competitive inhibition. Also, apart from the
presence of amines at least in quantities equivalent to the alcohols, other physical and
chemical influences of nearly equimolar quantities of water in the reaction medium could
reverse the imine formation particularly at higher temperatures. At this point it seems
noteworthy to mention that the largest asymmetric catalytic process of the synthesis of the
herbicide, (S)-Metolachlor, involves the asymmetric hydrogenation of an imine, which in
reductive aminations was found to be of comparatively low efficieny.13e
To establish a preference for the imine hydrogenation over the reductive amination,
the hydrogenations of various other imines were carried out, which in all cases showed
83 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
superior results over their corresponding reductive aminations. Hydrogenation of N-(4-
methoxybenzylidene)aniline with 0.02 mol% of IIIA showed a TOF of 11700 h-1 in the first
0.25 h giving rise to 98% yield of the desired product 3ba within one hour (Table 4.3, entry
5). At a temperature of 120 °C, the electron deficient N-(4-nitrobenzylidene)aniline was
subjected to hydrogenation with 0.1 mol% of IIIA as catalyst, in which the tertiary amine
7ka was observed as side product, so that a higher catalyst loading of 0.5 mol% was adopted,
Table 4.3. Hydrogenation of various imines.a
IIIA TOF Yield Entry Imine (5) Time (h) 3 (mol%) (h-1) (3, %)
1 0.02 3 3900b 97
2 5aa 0.02 < 1 8122 b 3aa 97c
3 0.02 2 4957 b 97d
4 0.02 < 4 2200 b 97e
5 5ba 0.02 < 1 11700 b 3ba 99
6 5ka 0.50 < 1 > 192 3ka 96f
7 5ac 0.1 < 3 333 3ac 99
84 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
8 5la 0.1 < 1 2070 b 3la 99
9 5da 0.1 < 3 1012 b, g 3da 97
10 5ah 0.1 < 1 2810 b, g 3ah 99
11 5ai 0.1 < 2 1555 b, g 3ai 98g
OMe
12 5aj N 0.02 < 1 7760 b, g 3aj 97
Cl
N 13 5bi 0.02 < 2 2425 3bi 98 MeO
14 5aj 0.10 < 1 950 3aj 95e
15 5ad 0.15 6 106 3ad 95e
16 5ak 0.15 8 79 3ak 95e
17 5ma 0.5 < 2 198 3ma 99e
a Unless otherwise mentioned, all reactions were carried out in THF under 50 bar H2 pressure at 90 °C; TOF b c d and yield by GC/MS. In the first 0.25 h. 100 equiv. of n-Bu4NBr was added. 50 equiv. of n-Bu4NBr was added. eXIIIA was used as catalyst. fReaction was carried out at 120 °C. gTOF by 1H NMR spectroscopy based on the consumption of imine.
85 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
which showed a TOF of > 192 h-1 and gave rise to a yield of 96% of the desired product 3ka
within one hour (Table 4.3, entry 6). Like in the corresponding cases of reductive aminations,
imines bearing an electron deficient amine part, like N-benzylidene-4-nitroaniline showed
better activities even at the lower temperature of 90 °C in comparison with imines with an
electron deficient aldehyde part (Table 4.3, entry 7). At this temperature of 90 °C, N-(4-
fluorobenzylidene)aniline could be smoothly converted to the corresponding amine with 0.1
mol% loading of IIIA revealing a TOF of 2070 h-1 in the first 0.25 h and a yield of >99% of
the desired product 3la in < 1 h (Table 4.3, entry 8). Under these conditions, N-(4- chlorobenzylidene)aniline showed a TOF of 1012 h-1 in the first 0.25 h giving rise to 97% yield of the desired product 3da in < 3 h (Table 4.3, entry 9). Under these conditions, N-
benzylidene-4-fluoroaniline showed a TOF of 2780 h-1 in the first 0.25 h giving rise to 99% yield of the desired product 3ah in < 1 h (Table 4.3, entry 10) whereas N-benzylidene-4- chloroaniline showed a TOF of 1555 h-1 in 98% yield of 3ai in < 2 h (Table 4.3, entry 11).
Under these conditions, but with a comparatively lower loading of 0.02 mol% of IIIA, an imine bearing electron rich amine part, N-benzylidene-4-methoxyaniline showed a TOF of
7760 h-1 in the first 0.25 h giving rise to a yield of 97% of the desired product 3aj in < 1 h
(Table 4.3, entry 12). N-(4-methoxybenzylidene)-4-chloroaniline showed under these
conditions and loadings, a TOF of > 2425 h-1 with a yield of 98% of the desired amine 3bi
within 2 h (Table 4.3, entry 13). With the same loading, but at the higher temperature of 120
°C, N-benzylidene-1-naphthylamine was converted with a TOF of > 950 h-1 in the first 0.25
giving rise to 95% yield of the desired product 3aj in < 1 h (Table 4.3, entry 14). Imines
bearing aliphatic amine parts, like N-benzylidene-1-hexylamine and N-benzylidene-
isobutylamine, gave yields of 95% of the desired amines 3ad and 3ak, respectively, in 5 h
and 8 h when a catalyst loading of 0.15 mol% of IIIA was adopted at a temperature of 120
°C (Table 4.3, entries 15 and 16). Hydrogenation of the ketimine, phenyl-(1-
86 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
phenylethylidene)amine furnished under the previous hydrogenation conditions and a loading
of 0.5 mol% of IIIA a TOF of > 198 h-1 and a yield of 98% of the desired N-(1- phenylethyl)aniline product in < 2 h (Table 4.3, entry 17).
4.2.3. Mechanistic Studies and Preparation of [Re(A)(Br)3(NO)][NEt4] and formation of
[Re(POP)(Br)2(NO)] (POP = A)
The complex IIIA is only sparingly soluble in THF even at a temperature of 90 °C. However, it was found to be more and sufficiently soluble in the presence of 50 equiv. of imine (N- benzylideneaniline). The stoichiometric reaction of complex IIIA (which constitutes CH3CN) with H2 led to the formation of the [Re]NH3 complex XVIA and the same reaction with benzonitrile and H2 produced the [Re]benzaldehydeimine complex XVIIA. Also, IIA or
IIIA were active in the hydrogenation of styrene in absence of a co-catalyst. From these
experiments we concluded that either a bromide ligand or one of the phosphine moieties had
dissociated during catalysis (Chapter 3). Applying a 10 mol% loading of IIIA, the
hydrogenation of the imine, N-benzylideneaniline was attempted under a H2 pressure of 10
bar and at 90 °C in dichloromethane run for 30 min, which showed complete consumption of
the imine. Analysis of the final reaction mixture showed the catalyst IIIA and its acetonitrile
derivative IIA in a ratio of 82:16 according to a 31P NMR spectrum along with traces of other
unidentified products. Thus, the formation of IIA from IIIA can be ruled out as a rate
limiting step.
We thought then to first explore whether a mechanism with the possibility of a
dissociation of one of the phosphine atoms of the sixantphos ligand could be operative under
the catalytic conditions. For this reason, the dibenzofuran monophosphine complex XIVD
was prepared and applied as a catalyst found to be much less active in the hydrogenation
reaction of imines when compared to IIIA. Also, though not so promising, the possibility of a
protonation of one of the phosphorus arms of the diphosphine ligand occurring in a
87 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
bifunctional manner across Re-P bond during catalysis concomitant with its dissociation was
examined by addition of 2 or 20 equiv. of DBFphos to this reaction using XIVD. However,
this did not improve the performance of the catalysis, instead retarded it. From the above
experimental observations, one can rule out the possibility of a phosphine dissociation as a
crucial step in the catalytic cycle.
The last possibility of a kinetic influence of the ligand on the catalytic reaction course
[15] was to check reversible bromide dissociation. The addition of 100 equiv. of n-Bu4NBr
with respect to IIIA was expected to retard the hydrogenation of N-benzylideneaniline.
However, an unprecedented great increase in reaction rate was observed; a TOF of 8122 h-1
was obtained in the first 0.25 h completing the reaction in less than 1 h using a loading of
0.02 mol% of IIIA at 90 °C (Table 4.3, entry 3). This reaction, when carried out with 50
-1 equiv. of n-Bu4NBr showed a lower activity with a TOF of 4957 h in the first 0.25 h (Table
4.3, entry 4). Addition of n-Bu4NBr was also seen to increase the activity in the reductive amination (Table 4.1, entry 10).
When 5 equiv. of n-Bu4NBr were added to IIIA in THF-d8, we observed the formation of a new singlet resonance at -2.3 ppm and this reaction in CD2Cl2 heated to 100
°C showed a new signal at 0.3 ppm in the 31P NMR spectrum. This reaction mixture was
Scheme 4.2. Reaction of IIIA with halides.
88 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
Figure 4.1. Molecular structure of XVIIIA. Anisotropic displacement parameters are depicted at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. Selcected bond lengths: XVIIIA: Re1-Br1: 2.580(1); Re1-Br2: 2.5688(9); Re1-O2: 2.196(5); VIIIA: Re1-I1: 2.7952(2); Re1-I2: 2.7976(2); Re1-O1: 2.221(2).
evaporated to dryness and extracted with THF. Yellowish brown coloured single crystals could be obtained suitable for X-ray diffraction, along with n-Bu4NBr. The X-ray diffraction analysis showed that the Sixantphos ligand of this compound XVIIIA was coordinated in a tridentate fashion with the O atom involved in bonding to the rhenium centre and the two bromides were found disposed trans to the phosphorus ligands (Figure 4.1). However, the yield of this reaction to XVIIIA in both THF and CH2Cl2 did not exceed greater than 55% even when 20 equiv. of n-Bu4NBr was used at 100 °C.
Since complex XVIIIA could not be isolated in pure form due to the presence of n-
Bu4NBr (chapter 4), we thought to convert IIIA to XVIIIA using Me4NBr, which would be easier for purification of XVIIIA, but this could not give the product even at a temperature of
100 °C, presumably due to its insolubility in organic solvents. However, when 5 equiv. of
31 Et4NBr were applied in CD2Cl2, 80-85% of the product ( P NMR: 0.1 ppm) was formed analogous to the one obtained with n-Bu4NBr in CD2Cl2. This compound when extracted with benzene could give rise to 77% yield of the tribromo anionic complex
89 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
[Re(A)(Br)3(NO)][NEt4] (XIXA). Interestingly, though the reaction was incomplete, dispensing XIXA in THF showed another peak in 31P NMR spectra with precipitation of
Et4NBr, but this compound was not XVIIIA. It is assumed that the THF co-ordinated complex similar to XVIIIA had formed. However this reaction could not be enhanced even at a higher temperature of 100 °C. Thus it was concluded that in the reaction of IIIA with n-
Bu4NBr in CD2Cl2 tetra-n-butylammonium
Figure 4.2. Molecular structure of XXA. Anisotropic displacement parameters are depicted at the 50% probability level. + Et4N , hydrogen atoms and solvent molecules are omitted for clarity.
analogue of XIXA was obtained which then transformed completely to XVIIIA when dispensed in THF. Interestingly, the isolated complex XIXA when dispensed in CD2Cl2 showed another peak at 25.9 in the 31P NMR spectra. Single crystals suiable for X-ray diffraction analysis were obtained when benzene was layered on it. This was analyzed to be the trichloroanionic complex [Re(A)(Cl)3(NO)][NEt4] (XXA) realizing the participation of dichloromethane in this reaction (Figure 4.2).
However, the formation of complexes XIIIA and XVIIIA where in these cases the sixantphos O atoms are coordinated trans to NO ligand in the presence of halide ions can be explained on the basis of an association-dissociation mechanism (Scheme 4.2). Addition of
90 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
halide ions to complex IIIA would generate anionic trihalorhenium complexes analogoues to
XIXA or XXA (Scheme 4.2). The observed dissociation of these anionic trihalorhenium complexes in some cases would be due to the increase in electron density on the metal centre, where the push pull interaction in these complexes weakens leading to the dissociation of the halide ligand trans to NO ligand and thus totally leaving the halogen salts.
However, a 0.05% loading of the diiodo complex complex XIIIA (chapter 2) in the
hydrogenation of N-benzylideneaniline at 50 bar H2 pressure and at a temperature of 90 °C
showed a TOF of 2200 h-1 in the first 0.25 h giving rise t 97% yield of the desired secondary
amine in 3 h (Table 4.3, entry 4). Thus this catalyst was less efficient when compared to
IIIA. When the dibromo complex IIIA (25.3 ppm in 31P NMR) and diiodo complex XIIIA
31 (23.1 ppm in P NMR) were dispensed in CDCl3 at room temperature, a new signal was also
observed at 24.5 ppm in the 31P NMR spectrum, which could be attributed to a rhenium complex bearing a bromide and an iodide ligand. Also, it is worth mentioning that an increase in activity was observed when IIIA was used in the catalytic styrene hydrogenation in the
presence of catalytic B(C6F6)3 when compared to the reaction without B(C6F6)3. This was
attributed to a capture of the dissociated bromide ion by this boron Lewis acid.14
The Hammet correlation was then checked with the series of aryl substituents H,
OMe, F and Cl in para position of both the benzylidene part and the amine part of N- benzylideneanilines (Figure 4.3). The plots of log(TOF) vs substituent constants (σ) gave slopes (ρ) -2.17 for the substituents on the benzylidene part and -1.41 for those in the amine part.15 The slopes represents the sensitivity constants, and those greater than unity indicated that the imine hydrogenation is highly sensitive to substituents. Also, a negative value of the
ρ indicated a positive charge build up during the reaction.
91 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Figure 4.3. Hammet plots of the hydrogenation reactions of N-benzylideneanilines with para substituents using IIIA. Black: benzylidene part; left to right: OMe, H, F, Cl. Red: amine part; left to right: OMe, H, F, Cl.
The bromide ligand trans to the nitrosyl ligand is comparatively strongly bounded to the rhenium centre due to a strong π push pull interaction in the Br-Re-NO axis. This is revealed in the reactivity of complexes of type II, III, as well as that of analogous complexes bearing other diphosphine ligands activated with excess triethylsilane and followed by subsequent reaction with ethylene to form complexes VII, as well as rhenium complexes bearing a substitution pattern with cis disposed ethylene and a H ligands where in all cases the bromide ligand trans to NO ligand was not affected (Chapter 2).
The reaction of IIIA with N-benzylideneaniline in CH2Cl2 at 90 °C furnished partial formation of the C complex IIA. This was expected, since acetonitrile is a stronger ligand than the imine. However, under the catalytic conditions, acetonitrile in stoichiometric amounts is either hydrogenated to imines or amines (Chapter 3) or replaced by the imine to be hydrogenated. The ability of such rhenium complexes to hydrogenate nitriles has already been documented.16 Evidence for imine coordination could be derived from the formation of the benzylideneamine coordinated complex XVIIA, when IIIA was subjected to H2 and
92 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
benzonitrile at 140 °C (chapter 3). Also, the formation of isomerized imine in the reductive amination reaction of hexanal with 1-hexylamine (Table 4.2, entry 10) indicates coordination of the imine during catalysis and subsequent β-hydride abstraction as well known for the classical Wilkinson or Osbon type hydrogenation of olefins by metal hydride catalysts. Thus, a structure XVIIA1 analogous to XVIIA would be formed through the coordinatively unsaturated species XXIA under catalytic conditions (Scheme 4.3). Though evidence for this structure XXIIA could not be directly drawn from experiments, it is the only possible intermediate at this stage. Both steric and electronic factors arising from the large bite angle
P O = P
Scheme 4.3. Proposed mechanism for the hydrogenation of imines catalyzed by complex IIIA.
93 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Sixantphos ligand would lead to distortion and poor orbital overlap between rhenium and the
bound bromide ligand which enhances the dissociation of the bromide ligand trans to this
diphosphine.16 The P1Re1P2 angle (bite angle, β) in the complex IIA is 97.01(3)° and that in
rhenium-imine complex XVIIA is 96.19(7)°, indeed prone for a large bite angle effect.
The species XXIIA1 is assumed to undergo dissociation of the bromide ligand trans
to the large bite angle diphosphine ligand forming the cationic intermediate XXIIA.14 This
species is assumed to undergo an isomerisation reaction leading to the formation of the
species XXIIIA. This is enhanced by the addition of halide ions in a similar fashion as
described in Scheme 4.2, and thus a direct access to XXIIIA from XVIIIA would take place.
The site trans to NO ligand, the most active site for a hydride ligand (more hydridic)17 and there by a fast insertion of polar substrates into the Re-H bond is thus accessible and the oxidative addition of H2 to XXIIIA forming the Re(III) species XXIVA seems to be preferred in this ligand position. A deuterium kinetic isotopic effect (k(H2)/k(D2)) of 2
indicated that the oxidative addition of H2 to be the possible rate limiting step. The
comparatively larger negative slopes of the Hammett plots for substituents on the aldehydic
and the amine part, that even much more prominent in the aldehydic part is expected to be
due to advancing the rhenium centre more electron rich through the N atom of the imine
which would enhance this oxidative addition of dihydrogen. Insertion of the coordinated
imine in to the Re-Htrans to NO bond followed by reductive elimination of the amine would
regenerate the species XXIA.
4.3. Conclusion
In summary, using rhenium complexes we have developed an efficient homogeneous catalysis for the reductive amination of aldehydes including α,β-unsaturated aldehydes showing excellent selectivities for the production of substituted amines. A highly efficient homogeneous hydrogenation of imines using one of the rhenium(I) complexes was also
94 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
discovered. Electron rich aromatics showed comparatively high activities in these processes.
Though the reaction was found to be very efficient, electron withdrawing substituents on the aldehydic part were much inferior when compared with these substituents on the amine part.
In this context we had also seen the ability of the given rhenium complexes to induce catalytic dehalogenations, as well as hydrogenation of aldehydes including formaldehyde to their corresponding alcohols. A reversible halide ligand dissociation to generate active species followed by a classical mechanism involving oxidative addition of dihydrogen, insertion of amine into the Re-H bond followed by reductive elimination is proposed for this hydrogenation reaction of imines. The ability of this rhenium complexes to hydrogenate ketimines further opens up the opportunity for ligand sphere tuning using appropriate chiral diphosphines thereby to impart highly efficient stereoselective hydrogenations.
4.4. Experimental Section
All manipulations of addition of reaction components and samplings were done in a glove box filled with dry
N2. All the reagents are purchased from either Aldrich or ABCR chemical company and used without further purification.
4.4.1. Preparation of [Re(A)Br3(NO)][NEt4] (XIXA)
Complex IIIA (100 mg, 0.099 mmol) and Et4NBr (104 mg, 0.494 mmol) was taken in a Young Schlenk flask and CH2Cl2 (1 mL) was added to it. It was heated to 100 °C for 1 h. The mass was cooled to room temperature, concentrated to dryness. The residue was extracted with benzene (2 x 1 mL), concentrated and dried to get the product XIXA as orange solid. Yield: (95 mg, 0.0761 mmol, 77%); IR (KBr, cm-1): 3053 (w), 2980 (w), 2946
(w), 1676 (s), 1482 (w), 1434 (m), 1378 (s), 1247 (w), 1228 (w), 1186 (w), 1095 (w); 1H NMR (500 MHz,
C6D6): δ 0.30 (s, 3H), 0.32 (s, 3H), 1.85 (t, 12H, J = 7.5 Hz), 3.02 (q, 8H, J = 7.5 Hz), 6.73-6.78 (m, 8H), 6.98-
31 1 7.05 (m, 6H), 7.26 (d, 2H, J = 7 Hz), 7.30-7.33 (m, 2H), 8.07 (m, 8H); P{ H}NMR (121 MHz, CDCl3): δ 0.1
(s); Anal. (%). Calc for C46H52Br3N2O2P2ReSi: C, 46.79; H, 4.44; N, 2.37. Found: C, 47.11; H, 4.57; N, 2.40.
4.4.2. Typical procedure for the direct reductive amination of aldehydes
Catalyst IIIA (0.002 g, 1.976 ×10-3 mmol) was taken in a stainless steel autoclave. Benzaldehyde (0.419 g,
95 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
3.953 mmol) and aniline (0.368 g, 3.953 mmol) were added followed by THF (1.5 mL). The autoclave was pressurized with 50 bar of hydrogen and kept in an oil bath maintained at 90 °C. After appropriate reaction time, the vessel was cooled to room temperature and the hydrogen was slowly released in a fume hood. Solvent was evaporated, redissolved in dichloromethane, filtered through a short plug of MgSO4 and the yield was measured by GC/MS based on the consumption of aniline.
4.4.3. Typical procedure for the hydrogenation of imines
Catalyst IIIA (0.002 g, 1.976 ×10-3 mmol) was taken in a stainless steel autoclave. N-benzylideneaniline (1.791 g, 9.881 mmol) was added followed by THF (3 mL). The autoclave was pressurized with 50 bar of hydrogen and kept in an oil bath maintained at 90 °C. After appropriate reaction time, the vessel was cooled to room temperature and the hydrogen was slowly released in a fume hood. The mass was filtered through a short plug of celite and the yield was measured by GC/MS based on the consumption of the imine.
GC/MS data of aldehydes, imines and amines: (MS (CP-3800 Saturn 2000MS/MS spectrometer, Column:
Brechbuhler, ZB-5ms, 30m x 0.25mm x 0.25µm) (compound: retention time, mass peak): 1a: 3.66 min (m/z =
106); 1b: 6.06 min (m/z = 136); 1c: 6.79 min (m/z = 151); 1d: 5.05 min (m/z = 140); 1e: 8.13 min (m/z = 156);
1f: 3.93 min (m/z = 112); 1h: 2.86 min (m/z = 100); 1j: 4.19 min (m/z = 132); 2a: 3.75 min (m/z = 93); 2b: 6.03 min (m/z = 219); 2c: 8.64 min (m/z = 138); 2d: 2.74 min (m/z = 101); 2f: 7.24 min (m/z = 169); 2g: 1.63 min
(m/z = 85); 3aa: 9.33 min (m/z = 183); 3ab: 10.28 min (m/z = 309); 3ac: 13.04 min (m/z = 307); 3ad: 8.02 min
(m/z = 191); 3ae: 4.02 min (m/z = 107); 3af: 10.11 min (m/z = 259); 3ag: 5.70 min (m/z = 175); 3ca: 13.19 min
(m/z = 228); 3da: 10.71 min (m/z = 217); 3ea: 13.93 min (m/z = 233); 3fa: 9.42 min (m/z = 189); 3ga: 6.15 min
(m/z = 149); 3hd: 6.63 min (m/z = 185); 3ia: 5.20 min (m/z = 107); 3if: 7.25 min (m/z = 183); 3ja: 11.30 min
(m/z = 209); 3ka: 13.04 min (m/z = 228); 3la: 9.43 min (m/z = 201); 3ma: 9.43 min (m/z = 197); 3ai: 10.84 min
(m/z = 217); 3bi: 13.70 min (m/z = 233); 3ak: 6.10 min (m/z = 149); 5aa: 9.07 min (m/z = 181); 5ab: 9.96 min
(m/z = 307); 5ba: 10.29 min (m/z = 215); 5ac: 12.11 min (m/z = 226); 5da: 10.29 min (m/z = 215); 5ea: 13.74 min (m/z = 257); 5fa: 9.07 min (m/z = 187); 5ga: 9.07 min (m/z = 147); 5ad: 7.95 min (m/z = 189); 5hd: 6.30 min (m/z = 183); 5ae: 9.07 min (m/z = 181); 5ia: 9.07 min (m/z = 181); 5ja: 10.63 min (m/z = 207); 5ka: 12.11 min (m/z = 226); 5la: 9.04 min (m/z = 199); 5ai: 10.37 min (m/z = 215); 5bi: 12.56 min (m/z = 245); 5aj: 13.30 min (m/z = 231); 5ad: 7.92 min (m/z = 189); 5ak: 6.06 min (m/z = 147); 5ma: 8.86 min (m/z = 195); 9hd: 7.00 min (m/z = 183); 10hd: 9.60 min (m/z = 265); 11a: 4.19 min (m/z = 106); 11b: 6.15 min (m/z = 138); 11d: 5.90 min (m/z = 142); 11e: 8.58 min (m/z = 158); 11f: 4.23 min (m/z = 114); 11h: 2.82 min (m/z = 102); 11j: 6.42 min (m/z = 136).
96 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
4.5. References 1 a) S. Gomez, J. A. Peters, T. Maschmeyer, Adv. Synth. Catal. 2002, 344, 1037-1057; b) B. Chen, U. Dingerdissen, J. G. E. Krauter, H. G. J. L. Rotgerink, K . Mobus, D. J. Ostgard, P. Panster, T. H. Riermeier, S. Seebald, T. Tacke, H. Trauthwein, Appl. Catal. A: Gen. 2005, 280, 17-46; A. Galan, J. de Mendoza, P. Prados, J. Rojo A. M. Echavarren, J. Org. Chem. 1991, 56, 452-454. a) R. N. Salvatore, C. H. Yoon, K. W. Jung, Tetrahedron, 2001, 57, 7785-7811; and see reference therein; b) M. Freifelder, Catalytic Hydrogenation in Organic Synthesis: Procedures and Commentary; (Wiley: New York, 1978; Chapter 10). a) B. Miriyala, S. Bhattacharyya, J. S. Williamson, Tetrahedron, 2004, 60, 1463-1471; b) A. Togni, L. M. Venanzi, Angew. Chem. Int. Ed. 1994, 33, 497-526; c) M. Sawamura, Y. Ito, Chem. Rev. 1982, 92, 857-871. 2. For reviews and recent examples; see, a) A. F. Abdel-Magid, S. J. Mehrman, Org. Process Res. Dev. 2006, 10, 971-1031 b) M. Tajbakhsh, H. Alinejad, M. Azarpira, M. Hosseinzadeh, H. Sadeghifara, S. Khaksar, Iran. J. Org. Chem. 2009, 2, 88-91; c) M. Tajbakhsh, R. Hosseinzadeh, H Alinezhad, S. Ghahari, A.Heydari, S. Khaksar, Synthesis 2011, 3, 490-496; J. Han, V. Tschernutter, J. Yang, T. Eckle, C. H. Borchers, Anal. Chem. 2013, 85, 5965-5973; d) R. Lokhande, J. Sonawane, A. Roy, L. Ravishankar, Green Chem. Lett. Rev. 2011, 4, 69-72; e) P. V. Ramachandran, P. D. Gagare, K. Sakavuyia, P. Clark, Tetrahedron Lett. 2010, 51, 3167-3169; f) J. E. Grob, J. Nunez, M. A. Dechantsreiter, L. G. Hamann, J. Org. Chem. 2011, 76, 4930-4940 g) O. Y.Wong, A. E. Mulcrone, S. K. Silverman, Angew. Chem. Int. Ed. 2011, 50, 11679-11684; h) N. U. Kumar, B. S. Reddy, V. P. Reddy, R. Bandichhor , Tetrahedron Lett. 2012, 53, 4354-4356; i) G. A. Molander, D. J. Cooper, J. Org. Chem. 2008, 73, 3885-3891; k) F. I. McGonagle, D. S. MacMillan, J. Murray, H. F. Sneddon, C. Jamiesona, A. J. B. Watson, Green Chem. 2013, 15, 1159; l) J. Nöth, K. J. Frankowski, B. Neuenswander, J. Aubé, O. Reiser, J. Comb. Chem. 2008, 10, 456-459; m) R. Neelarapu, P.A. Petukhov, Tetrahedron 2012, 68, 7056-7062. n) H. Firouzabadia, N. Iranpoora, H. Alinezhad, J. Iran. Chem. Soc. 2009, 6, 177-186; o) E. E. Boros, J. B. Thompson, S. R. Katamreddy, A. J. Carpenter, J. Org. Chem. 2009, 74, 3587-3590; p) H. Alinezhad, M. Tajbakhsh, F. Salehian, K. Fazli, Tetrahedron Lett. 2009, 50, 659-661; q) E. M. Dangerfield, C. H. Plunkett, A. L. Win- Mason, B. L. Stocker, M. S. M. Timmer, J. Org. Chem. 2010, 75, 5470-5477; r) Md. W. Ahmad, S. Y. Lee, T. J. Kim, H-S Kim, Bull. Korean Chem. Soc. 2011, 32, 4079-4082; s) S. Chandrasekhar, V. M. Rao, Tetrahedron: Asymmetry 2012, 23, 1005-1009; t) W. Liao, Y. Chen, Y. Liu, H. Duan, J. L. Petersen, X. Shi, Chem. Commun., 2009, 6436-6438. 3. a) T. Mizuta, S. Sakaguchi, Y. Ishii J. Org. Chem. 2005, 70, 2195-2199; b) R.-Y Lai, C.-I Lee, S.-T Liu, Tetrahedron 2008, 64 1213-1217; c) R. Apodaca, W. Xiao, Org. Lett. 2001, 3, 1745. d) O.-Y Lee, K.-L Law, D. Yang, Org. Lett. 2009, 11, 302-3305. e) O. -Y Lee, K-L Law, C.-Y Ho, D. Yang, J. Org. Chem. 2008, 73, 8829-8837; f) P. D. Pham, P. Bertus, S. Legoupy, J. Chem. Soc., Chem. Commun. 2009, 6207- 6209; g) S. C. A. Sousaa, A. C. Fernandes, Adv. Synth. Catal. 2010, 352, 2218-2226; h) B. G. Das, P. Ghorai, Chem. Commun. 2012, 48, 8276-8278; i) B. G. Das, P. Ghorai, 2012, 48, 8276-8278; j) S. Enthaler, Catal. Lett. 2011, 141, 55-61. 4. F.-M. Gautier, S. Jones, X. La, S. J. Martin, Org. Biomol. Chem., 2011, 9, 7860-7868. 5. a) B.-C. Chen, J. E. Sundeen, P. Guo, M. S. Bednarz, R. Zhao, Tetrahedron Lett. 2001, 42, 1245-1246.
97 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
6. a) T. Suwa, E. Sugiyama, I. Shibata, A. Baba, Synlett 2000, 556-558; b) I. Shibata, T. Moriuchi- Kawakami, D. Tanizawa, T. Suwa, E. Sugiyama, H. Matsuda, A. Baba, J. Org. Chem. 1998, 63, 383-385; c) I. Shibata, T. Suwa, E. Sugiyama, A. Baba, Synlett 1998, 1081-1082; d) T. Suwa, I. Shibata, K. Nishino, A. Baba, Org. Lett. 1999, 1, 1579; e) T. Suwa, E. Sugiyama, I. Shibata, A. Baba, Synthesis 2000, 789-800. 7. a) I. V. Micovic, M. D. Ivanovic, D. M. Piatak, V. D. Bojic, Synthesis 1991, 1043-1045. b) A. da S. Renato, I. H. S. Estevamb, L. W. Biebera Tetrahedron Lett. 2007, 48, 7680-7682; c) K. Ushikoshi, K. Mori, T. Watanabe, M. Takeuchi, M. Saito, M. Stud. Surf. Sci. Catal. 1998, 114, 357; d) M. Saito, Catal. Surv. Jpn. 1998, 175; e) L. C. Grabow, M. Mavrikakis, ACS Catal. 2011, 1, 365. 8. a) B. Basu, S. Jha, M. M. H. Bhuiyan, P. Das Synlett 2003, 4, 555-557; b) N. A. Strotman, C. A. Baxter, K. M. J. Brands, E. Cleator, S. W. Krska, R. A. Reamer, D. J. Wallace, T. J. Wright, J. Am. Chem. Soc.2011, 133, 8362-8371; c) Q. Lei, Y. Wei, D. Talwar, C.Wang, D. Xue, J. Xiao, Chem. Eur. J. 2013, 19, 4021-4029. 9. a) T. Itoh, K. Nagata, A. Kurihara, M. Miyazaki A. Ohsawa, Tetrahedron Lett. 2002, 43, 3105-3108; b) D. Menche, J. Hassfeld, J. Li, G. Menche, A. Ritter, S. Rudolph, Org. Lett., 2006, 8, 741-744; c) M. Zhang, H. Yang, Y. Zhang, C. Zhu, W. Li, Y. Cheng, H. Hua; Chem. Commun., 2011, 47, 6605–6607; d) V. N. Wakchaure, M. Nicoletti, L. Ratjen, B. List, Synlett 2010, 18, 2708-2710; e) V. N. Wakchaure, J. Zhou, S. Hoffmann, B. List, Angew. Chem. Int. Ed. 2010, 49, 4612-4614; f) R. I. Storer, D. E. Carrera, Y. Ni, D. W. C. MacMillan, J. Am. Chem. Soc. 2006, 128, 84-86; g) Q. P. B. Nguyen, T. H. Kim, Synthesis, 2012, 44, 1977–1982; h) Q. P. B. Nguyen, T. H. Kim, Tetrahedron 2013, 69, 4938-4943. 10. K. Saito, T. Akiyama, Chem. Commun., 2012, 48, 4573–4575. 11. a) A. Robichaud, A. N. Ajjou, Tetrahedron Lett. 2006, 47, 3633–3636; b) M. D. Bhor, M. J. Bhanushali, N. S. Nandurkar, B. M. Bhanage, Tetrahedron Lett. 2008, 49, 965–969. 12. a) M. H. S. A. Hamid, P. A. Slatford, J. M. J. Williams, Adv. Synth. Catal. 2007, 349, 1555-1575; b) R. N. Salvatore, C. H. Yoon and K. W. Jung, Tetrahedron 2001, 57, 7785; and see reference there in; c) M. Freifelder, in Catalytic Hydrogenation in Organic Synthesis: Procedures and Commentary; Wiley, New York, 1978; Ch. 10. 13. a) V. I. Tararov, R. Kadyrov, T. H. Riermeier, A. Börner, Chem. Commun. 2000, 1867–1868; b) J. G. de Vries, C. J. Elsevier, Handbook of Homogeneous Hydrogenation, Vol. 1; (Wiley, Weinheim, 2007; Chapter 15, pp 437-439); c) K.S. Hayes, Appl. Catal. A: Gen. 2001, 221, 187-195; d) L. L. Mark′o, J. Bakos, J. Organomet. Chem. 1974, 81, 411-414; e) H -U. Blaser, H -P. Buser, H-P. Jalett, B. Pugina, F. Spindler, Synlett 1999, 867–868; f) T. C. Nugent, M. El-Shasly, Adv. Synth. Catal. 2010, 352, 753-819; g) J. F. Kniffon, Catal. Today 1997, 36, 305; h) T. Gross, A. M. Seayad, M. Ahmad, M. Beller, Org. Lett. 2002, 12, 2055-2058; i) S. Werkmeister, K. Junge, M. Beller, Green Chem., 2012, 14, 2371-2374; j) A. Pagnoux-Ozherelyeva, N. Pannetier, M. D. Mbaye, S. Gaillard, J- L Renaud, Angew. Chem. Int. Ed. 2012, 51, 4976-4980; k) S. Fleischer, S. Zhou, K. Junge, M. Beller, Chem. Asian J, 2011, 6, 2240-2245: l) B. Villa-Marcos, C. Li, K. R. Mulholland, P. J. Hogan, J. Xiao, Molecules 2010, 15, 2453-2472; m) C. Li, B. Villa-Marcos, and J. Xiao, J. Am. Chem. Soc. 2009, 131, 6967-6969; n) Y. Chi, Y -Gui Zhou, X. Zhang, J. Org. Chem. 2003, 68, 4120-4122; o) L. Rubio-Pe´rez, F. J. Pe´rez-Flores, P. Sharma, L. Velasco, A. Cabrera, Org. Lett., 2009, 11, 265-268; p) V. I. Tararov, R. Kadyrov, T. H. Riermeier, A. Börner, Adv.
98 Chapter 4 Rhenium Catalyzed Highly Efficient Homogeneous Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Based on Reversible Halide Dissociation
Synth. Catal. 2002, 344, 200-208; q) G. F. Busscher, L. Lefort, J. G. O. Cremers, M. Mottinelli, R. W. Wiertz, B. de Lange, Y. Okamura, Y. Yusa, K. Matsumura, H. Shimizu, J. G. de Vries, A. H. M. de Vries, Tetrahedron: Asymmetry 2010, 21, 1709-1714; r) O. Bondarev, C. Bruneau, Tetrahedron: Asymmetry 2010, 21, 1350-1354. 14. Y. Jiang, J. Hess, T. Fox, H. Berke, J. Am. Chem. Soc. 2010, 133, 18233-18247. 15. L. P. Hammet, J. Am. Chem. Soc. 1937, 59, 96-103. 16. a) K. A. Lenero, M. Kranenburg, Y. Guari, P. C. J. Kamer, P. W. N. M. van Leeuwen, S. Sabo-Etienne, B. Chaudret, Inorg. Chem. 2003, 42, 2859-2866; b) P. C. J. Kamer, P. W. N. M. Van Leeuwen, J. N. H. Reek, Acc. Chem. Res. 2001, 34, 895-904. 17. a) R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 2134; b) R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 4450; c) R. H. Crabtree, A. Gautier, G. Giordano, and T. Khan, J. Organometal. Chem. 1977, 141, 113; d) H. Berke, P. Burger, Comments Inorg.Chem. 1994, 16, 279-312; e) H. Jacobsen, H. Berke, in Recent Advances in Hydride Chemistry; (Ed.: R. Poli), Elsevier: Amsterdam, Holland, 2001; pp 89-116; f) A. Choualeb, E. Maccaroni, O. Blacque, H. W. Schmalle, H. Berke, Organometallics 2008, 27, 3474-3481.
99 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Homogeneous Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol Catalyzed by Rhenium Complexes
5.1. Introduction
The inexpensive, abundant, of low toxicity CO2 gas is one of the main originators for green house effect and thereby leading to global warming and climatic changes.1 The burning of fossil fuels to serve the world’s energy demands has led to accumulation of this gas in the atmosphere.2 The future energy demand relies on sustainable or renewable energy sources, since it is estimated that the fossil fuel sources will deplete in the near future.3 In this context, production of synthetic fuel from environmentally harmful carbon sources would be a method of choice. For that reason the hydrogenation of carbon dioxide to methanol, a C1 feed stock, is highly demanded to run the daily needs of the future, thereby to recycle CO2 and to reduce
4 CO2 emissions.
Hydrogenation of CO2 to methanol has been reported with heterogeneous catalytic systems. Most prominent among them are the Cu-Zn based systems that operate at relatively high temperatures and pressures.5 These processes limit the advantages of tuning the catalytic systems to be operateed at ambient conditions with high efficiency and selectivity. On the other hand, the homogeneous catalytic systems offer opportunity for ligand sphere turning thereby imparting it to be operational at ambient conditions and with excellent selectivities.6
Remarkable achievements in industrial processes have evoked from the concept of homogeneous transition metal catalysis, but they are dominated by scarce precious metals, like Ru, Rh, Pd and Ir.7 In the recent past, tremendous efforts were made in academic
100 Chapter 5 Homogeneous Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol Catalyzed by Rhenium Complexes
research to develop catalytic systems that are capable of reducing CO2 to formates using well defined Ru, Rh and Ir systems.8 Efficient hydrogenations of formate esters and organic
carbonates to methanol were realized using Milstein’s pincer type ruthenium catalysts.9
Reduction of CO2 to methanol has been achieved using boranes, phosphaboranes and silanes,
but apart from the issue of high costs, these reagents lead also to the formation of large
amount of waste.10 Preliminary outcome of the efforts on metal catalyzed homogeneous reduction of CO2 to methanol was reported recently by Huff and Sanford through ruthenium catalyzed cascade reaction involving formic acid and methyl formate as intermediates.11 In
this report, different ruthenium complexes, which are reported to be capable of catalyzing
CO2 to the formate level, and formate esters to methanol were rationally sequenced along
with the presence of an acid co-catalyst, the latter was added to enhance esterification and all
carried out in a single reaction vessel to effect this transformation. Quite recently, Leitner and
co-workers demonstrated this reaction using a ruthenium-phosphine catalytic system, which
was already found to be efficient for the hydrogenation of carboxylic acids and their
derivatives to the corresponding alcohols.12 In either of these cases, suitable additives were necessary to perform these reactions. Also, all these catalytic systems were to be handled under air and moisture free conditions.
5.2. Results and Discussion
5.2.2. Catalytic Hydrogenation of Carbon Dioxide to Methanol
In the recent past, our group has developed highly efficient homogeneous rhenium based hydrogenation catalysts, some of which showed activities comparable to those of precious metals in hydrogenations of olefins. CO2 reduction assisted by rhenium
14 hydride/B(C6F6)3 ‘Frustrated Lewis Pair’ has been recently documented as well. Based on the diphosphine nitrosyl rhenium complexes discussed in the previous chapters, we describe
101 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Table 5. 1. Hydrogenation of carbon dioxide to methanol catalyzed by various rhenium complexesa
Co-catalyst 2 Entry [Re] Co-catalyst 1 /Equiv. CO /H (bar) TONb /Equiv. 2 2
1 IIIA - - 10/30 07
2 IIIA n-Bu4NBr /5 - 10/30 28
c 3 IIIA n-Bu4NBr /5 - 10/30 26
4 XIXA - - 10/30 07
5 XIXA - - 10/30 00d
6 XIXA - - 10/30 00e
7 XIIIA - - 10/30 13
8 IIIA n-Bu4NI /100 - 10/30 11
9 IIIA NaI/100 - 10/30 05
10 IIIA n-Bu4NBr /2 - 10/30 20
11 IIIA n-Bu4NBr /50 - 10/30 30
12 IIIA n-Bu4NBr /100 - 10/30 30
13 IIIA Et4NBr /100 - 10/30 1.6
14 XIIIA n-Bu4NBr /100 - 10/30 33
f 15 IIIA n-Bu4NBr /5 - 10/30 0.9
16 IIIA - EtOH/250 10/30 06
17 IIIA n-Bu4NBr /100 EtOH/250 10/30 33
g 18 IIIA n-Bu4NBr /5 EtOH/100 10/30 29
19 XXVA - - 10/30 20
20 IIIA - - 20/60 12
21 IIIA n-Bu4NBr /5 - 20/60 88
22 XIVD - - 10/30 00 a0.005 mmol of catalyst, 1 mL of solvent, TON by 1H NMR spectroscopy using DMF as internal standard; b c d e moles of MeOH/moles of Re; Reaction components were charged in open air. H2O as solvent; Toluene as f g solvent; 1:1 mixture of H2O:THF as solvent; 5 equiv. of p-TsOH;
102 Chapter 5 Homogeneous Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol Catalyzed by Rhenium Complexes
herein the production of methanol via homogeneous hydrogenation and hydrosilylation of
CO2. Initial testing and optimization of the hydrogenation reaction using rhenium complexes
were carried out at a CO2 pressure of 10 bar and a H2 pressure of 30 bar at 140 °C in THF run
for 18 h (Table 5.1). The rhenium complex IIIA alone under these conditions gave methanol
with a TON of 7 quantified by 1H NMR spectroscopy using DMF as an internal standard
(Table 5.1, entry 1). The formation of methanol is further confirmed by 31C NMR spectroscopy and again by adding a little of methanol to this mixture followed by further analysis. Since the activity of this complex in hydrogenations was found to increase by the addition of n-Bu4NBr and due to the formation of complex XVIIIA (Chapter 4), we added 5 equiv. of n-Bu4NBr with respect to the catalyst and performed the hydrogenation of CO2. We could achieve a TON of 28 under the above mentioned conditions of optimization (Table 5.1, entry 2). It is worth mentioning that the starting components of this reaction could be handled in open air, which gave comparable results with those reactions handled in a glove box (Table
5.1, entry 3). The tribromo anionic complex XIXA as a catalyst gave only a TON of 7 under the above hydrogenation conditions (Table 5.1, entry 4). It is worth mentioning that the complex XIXA could not give any product when the reaction was carried out in water or
toluene (Table 5.1, entries 5 and 6). The diiodo complex XIIIA was found to produce
methanol with a TON of 13 under the above hydrogenation conditions whereas IIIA with
presence of 100 equiv. of n-Bu4NI with respect to IIIA allowed TON of 11 (Table 5.1,
entries 7 and 8). However, complex IIIA along with 100 equiv. of NaI in THF provided a
TON of only 5 (Table 5.1, entry 9). Addition of only 2 equiv. of n-Bu4NBr along with IIIA
could give a TON of 20, which is a little less in quantity of methanol when compared to the
reaction with 5 equiv (Table 5.1, entry 10). However, addition of 50 or even 100 equiv. of n-
Bu4NBr to this reaction gave TONs comparable to the reaction that with 5 equiv. (Table 5.1,
entries 11 and 12). From these observations, it was concluded that the role of n-Bu4NBr that
103 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
under catalytic conditions was to convert complex IIIA to XVIIIA under the. However, the reaction using IIIA along with 100 equiv. of Et4NBr was seen to give methanol in a TON of only 1.6 (Table 5.1, entry 13). This reduced activity is anticipated to be due to the increased formation of complex XIXA which is inactive in catalysis unless it dissociates bromide (as
Et4NBr) in THF as discussed in Chapter 5. Addition of 5 equiv. of n-Bu4NBr along with
XIIIA provided the higher TON of 33 (Table 5.1, entry 14). A reaction in a 1 : 1 mixture of
H2O : THF was seen to provide a TON of only 0.9 (Table 5.1, entry 15), for reasons which cannot be explained as yet.
In order to enhance the formation of the first reduced product, formic acid, by stabilizing it as formate ester, 250 equiv. of ethanol was added to the reaction mixture using
IIIA as catalyst and no other additive. However, this did not lead to an improved TON (Table
5.1, entry 16). Also, this reaction of addition of ethanol in the presence of n-Bu4NBr did not show any significant improvement when compared to the reaction without the addition of ethanol (Table 5.1, compare entries 12 and 17). The hydrogenation reaction in the presence of
100 equiv. of ethanol and 5 equiv. of p-TsOH to enhance the ester formation also could not provide any significant increase in TON (Table 5.1, entry 18). Also, ethyl formate or methyl formate could not be observed in 1H NMR spectra, even not in traces.
Now, on attempts to synthesize a Re-formate complex, IIIA was reacted with with 5 equiv. of sodium formate in acetone (laboratory grade) at 55 °C. This revealed a dinuclear
3 trihydroxy complex [Re2(A)2µ -(OH)3(NO)2][Br] (XXVA) (Figure 5.1). However, this complex could not be obtained when dry acetone was used, instead the formation of various other species were observed which require further characterization. Quite surprisingly, the µ- hydroxy compound XXVA could not be obtained when water or NaOH was reacted instead of sodium formate. However, according to 31P NMR spectrum the formation of other
104 Chapter 5 Homogeneous Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol Catalyzed by Rhenium Complexes
H + ON NO O P - P Re Re Br O O O P H P O H
Figure 5.1. Complex XXVA (left) and Molecular structure of it (right). Anisotropic displacement parameters are depicted at the 50% probability level. Ph groups on P atoms, Br-, hydrogen atoms except those on bridging O and solvent molecules are omitted for clarity.
unidentified species was prevailing. Thus, at this stage, we conclude that water led to
formation of product XXVA from IIIA. In hydrogenation experiments to yield methanol,
XXVA as a catalyst provided a TON of 20 under the above mentioned reaction conditions
(Table 5.1, entry 19).
Tuning the methanol production under relatively mild conditions, further we thought
to carry out these reactions under pressures of 20 bar CO2 and 60 bar H2 at the temperature of
140 °C and run for 18 h. The reaction using IIIA as a catalyst gave methanol with TON of 12
(Table 5.1, entry 20). Another run of this reaction along with 5 equiv. of n-Bu4NBr furnished
methanol with a TON of 88 (Table 5.1, entry 21). However, the same reaction conditions
using the monophosphine complex XIVD did not show hydrogenation of CO2 (Table 5.1, entry 22), which emphasized the necessity for large bite angle diphosphines in the coordination sphere of rhenium.
5.2.2. Hydrosilylation and Combined Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol Using Complexes IIIA and XVIIA.
Based on our previous studies to improve the activity of rhenium Sixantphos nitrosyl
105 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
complexes including IIIA along with Et3SiH as a co-catalyst for the hydrogenation of olefins
(Chapter 2) and nitriles (Chapter 3), we thought to apply this strategy for this hydrogenation of CO2 to methanol. Complex IIIA along with 25 equiv. of Et3SiH furnished under the conditions of 10 bar CO2 and 30 bar H2 in THF run for 18 h MeOH in a TON of 16 when quenched in D2O. This reaction with the addition of 5 equiv. of n-Bu4NBr furnished methanol in a TON of 51 (Table 5.2, entries 1 and 2). In these cases of catalysis, both hydrogenation and hydrosilylation reactions are combined to faster the formation of methanol (Scheme 5.1).
A run of this reaction using 100 equiv. of Et3SiH was carried out along with 5 equiv. of n-
Bu4NBr showed production of methanol with a TON of 37 and MeOSiEt3 with a TON of 27 when samples were taken in a glove box and analyzed by 1H NMR spectroscopy using dry
Table 5.2. Hydrosilylation/hydrogenation of carbon dioxide to the methanol level catalyzed by complex IIIA a and XIIIA along with Et3SiH
b Entry [Re] Co-catalyst/Equiv. Et3SiH (Equiv.) CO2/H2 (bar) TON
1 IIIA - 25 10/30 16
2 IIIA n-Bu4NBr /5 25 10/30 51
c 3 IIIA n-Bu4NBr /5 100 10/30 64
4 IIIA n-Bu4NBr /5 100 20/60 130
d 5 IIIA n-Bu4NBr /5 100 20/60 330
6 XIIIA n-Bu4NBr /5 100 20/60 92
7 IIIA - 40 02/0 3.2
8 IIIA n-Bu4NBr /5 1000 10/0 160
e 9 IIIA n-Bu4NBr /5 1000 20/0 420 a0.005 mmol of catalyst and 1 mL of THF at 140 °C run for 18 h; for entry 7, reaction was carried out at 80 °C and run for 10 h, TON by 1H NMR spectroscopy sing DMF or mesitylene as internal standard; bMeOH: d e 37; MeOSiEt3: 27. Reaction was run for 96 h. 2 equiv. of B(C6F6)3 with respect to the catalyst was added.
106 Chapter 5 Homogeneous Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol Catalyzed by Rhenium Complexes
Scheme 5.1. Reaction pathways in the hydrogenation/hydrosilylation of CO2 using IIIA.
1 THF-d8. This reaction mixture when quenched with D2O followed by H NMR analysis showed methanol with a TON of 64 (Table 5.2, entry 3). A batch of this reaction along with
100 equiv. of Et3SiH at pressures of 20 bar of CO2 and 60 bar of H2 was seen to give methanol in a TON of 130 (Table 5.2, entry 4). Another run of this reaction for 96 h produced methanol with a TON of 330 indicating longevity of the active species (Table 5.2, entry 5).
However, this reaction using complex XIIIA could give a TON of only 92 in 18 h (Table 5.2, entry 6).
An experiment in a Young NMR tube consisting of 0.005 mmol of IIIA along with
50 equiv. of Et3SiH in THF-d8 was charged with 2 bar CO2 revealing the formation of
H(CO)OSiEt3 and MeOSiEt3 with TONs of 0.90 and 0.74, respectively, when heated to 80 °C for 1 h. Continuing this reaction for 10 h gave these products in TONs of 0.36 and 3.22 respectively along with traces of CH2(OSiEt3)2 (Table 5.2, entry 7) (Scheme 5.1). A reaction carried out with 1000 equiv. of Et3SiH and 5 equiv. of n-Bu4NBr at a CO2 pressure of 20 bar at 140 °C in THF run for 18 h showed the formation of MeOSiEt3 with TON of 160 (Table
5.2, entry 8). This reaction with the addition of 2 equiv. of B(C6F6)3 with respect to IIIA gave
MeOSiEt3 with a TON of 420 (Table 5.2, entry 8). Reaction between B(C6F6)3 and silane is
+ expected to generate the silylium ion [Et3Si] eventually adding to the NO ligand and inducing nitrosyl bending.13a Thus the quite substantial increase in activity is attributed to be
107 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
a consequence of the generation of vacant sites, as well as the enhancement of splitting of
+ dihydrogen in a bifunctional manner across [Re-NOSiR3] bond.
5.2.3. Mechanistic Studies
To understand the mechanism of these reactions, we refer to Chapter 2. The bromide ligand
trans to the nitrosyl ligand is comparatively strongly bonded to the rhenium centre due to a
strong push pull interaction of the Br-Re-NO axis. This is revealed in the reactivity of IIIA
with Et3SiH, which led to formation of the rhenium silane dihydride species IVA, analyzed in
solution (Scheme 5.2). This reacted with 2 bar ethylene gave rise to a ɳ2-ethylene coordinated
complex forming stable 4-membered rhenacycles through the activation of a ortho-C-H of
one of the phenyl groups attached to a phosphorus atom, presumably due to the incapability
of the rhenium hydride bond to be stabilized trans to the large bite angle sixantphos ligand.
These rhenacycles where found to react with hydrogen to form H-bridged dinuclear
complexes of type X, from which the coordinatively unsaturated rhenium monohydride
monomer V was thought to be released as the active species. Analogous complexes bearing comparatively smaller bite angle diphosphine ligands than Sixantphos furnished rhenium complexes bearing an ethylene ligand and a H disposed cis to each other. However, in either
case, the bromide ligand trans to NO ligand was not affected. However, in the presence of n-
Bu4NBr, complex XVIIIA can be formed where the site trans to NO ligand is accessible
(Scheme 5.2).
Combining these experimental evidences and facts, a Re(III) species XXVIA is
assumed to be formed when IIIA is reacted with n-Bu4Br and Et3SiH (Scheme 5.2). This
species would be in equilibrium with the coordinatively unsaturated 16e [Re]-H species
XXVIIA analogous to the equilibrium between IVA and VA (Chapter 2). CO2 insertion into
the Re-H bond of XXVIIA would give rise to the coordinatively unsaturated formate species
108 Chapter 5 Homogeneous Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol Catalyzed by Rhenium Complexes
NO Br NO NO Et3SiH P H P Br P P Br (excess) n-Bu4NBr O Re SiEt3 O Re Re O Re P Br H P Br P P Br IVA Br NO O . 2 CH3CN + Et SiH - Et SiH XVIIIA 3 3 IIIA NO P Et3SiH n-Bu NBr O Re H (excess) 4 P Br VA P NO H O Re H P H SiEt3 XXVIA
+Et SiH -Et SiH 3 3 As perscheme 1
NO P O O O Re H P H H OSiEt XXVIIA O 3 H OSiEt3 Et SiH 3 + CO2 H OH H2 NO NO P NO P X Et SiH O Re X P X 3 H2 O Re H O Re H P P SiEt O P H O 3 O XXXA H XXVIIIA H XXIXA H O O O
Scheme 5.2. Proposed mechanism of hydrosilyation/hydrogenation of CO2 to silylformate using complex IIIA.
XXVIIIA. This can react with either H2 or Et3SiH to form species XXIXA and XXXA, respectively. Upon reductive elimination of H(CO)OH would regenerate the active species
XXVIIA. The formic acid elimination is favoured due to the formation of H(CO)OSiEt3 releasing H2. However, reductive elimination of H(CO)OSiEt3 rather than H(CO)OH is apparently preferred from XXXA, since no hydrogen could be observed in the hydrosilylation route. The formed H(CO)OSiEt3 could undergo further hydrogenation as well as hydrosilylation step as depicted in scheme 5.1.
5.3. Conclusion
In conclusion, homogeneous hydrogenation and hydrosilylation of CO2 to methanol could be
109 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
realized using suitable large bite angle diphosphine nitrosyl rhenium complexes. A preliminary mechanism for the Et3SiH and n-Bu4NBr co-catalyzed hydrogenation/hydrosilylation is proposed with formation of a crucial coordinatively unsaturated rhenium dihydride as the active species. This catalysis, particularly the air stable variant of these hydrogenations would impart opportunity for the development of efficient robust catalytic systems for these important transformations.
5.4. Experimental Section
Unless mentioned, all manipulations of addition of reaction components and samplings were done in a glove box filled with dry N2. All the reagents are purchased from either Aldrich or ABCR chemical company and used without further purification.
3 + - 5.5.1. Preparation of [Re2(A)2µ -(OH)3(NO)2] Br (XXVA)
Complex IIIA (20 mg, 0.0198 mmol) and sodium formate (15.93 mg, 0.0988 mmol) was taken in a Young
NMR tube and acetone (LR grade, 0.2 mL) was added to it. It was heated to 50 °C for 2 h. The mass was cooled to room temperature, concentrated to dryness. The residue was extracted with CH2Cl2 (2 x 0.5 mL), concentrated and dried to get the product XXIA as off-white to pale gray solid. The reaction leading to this product could not be reproduced even with dry or moist acetone and/or with NaOH since attempts gave rise to inseparable mixture of products. Yield: (12.47 mg. 0.0071 mmol, 72%; IR (KBr, cm-1): 3466 (br, m), 3409 (br, m), 3047 (w), 2924 (w), 1686 (s), 1560 (w), 1434 (m), 1380 (s), 1247 (w), 1228 (w), 1121 (w), 1090 (w); 1H
NMR (500 MHz, CDCl3): δ 0.38 (s, 3H), 0.63 (s, 3H), 0.65 (s, 3H), 1.33 (s, 3H), 5.59 (t, 4H, J = 7.5 Hz), 6.13
(t, 2H, J = 9 Hz) , 6.47 (t, 4H, J = 10.2 Hz), 6.58-6.62 (m, 4H), 6.67 (m, 6H), 6.90 (t, J = 8.3 Hz, 2H), 7.05 (t, J
= 7.0 Hz, 2H), 7.09 (t, J = 7.5 Hz, 2H), 7.22 (t, J = 7.3 Hz, 4H), 7.32 (t, J = 7.3 Hz, 2H), 7.48-7.56 (m, 6H),
31 1 7.65 (d, J = 7.5 Hz, 4H), 8.19 (d, J = 7.0 Hz) 4H); P{ H} NMR (121 MHz, CDCl3): δ 3.0 (s), 14.4 (s); Anal.
(%). Calc for C76H67BrN2O7P4Re2Si2: C, 52.08; H, 3.85; N, 1.60. Found: C, 50.81; H, 4.15; N, 1.37 (not satisfactory due to the presence of inseparable impurities).
5.5.2. Hydrogenation of Carbon Dioxide
Complex IIIA (5 mg, 0.00494 mmol) and Bu4NBr (7.96 mg, 0.0247 mmol) was taken in a stainless steel autoclave. THF (1 mL) was added to it. The vessel was pressurized with 20 bar of CO2 (the pressure decreased to 18 bar) and then with 60 bar of H2 (Total to 78 bar). It was heated to 140 °C for 18 h. The vessel was cooled
110 Chapter 5 Homogeneous Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol Catalyzed by Rhenium Complexes
to 0° C using an ice bath. The gases were slowly wented in to a fume hood. Immediately after opening the vessel, 10 µL of DMF was added to it as an internal standard and shaken well. Little of this solution (approx.
µL) was taken using a pipette and placed the pipette in an NMR tube followed by immediate dilution of this solution with appropriate quantity of D2O injected through the same pipette. The content of methanol is quantified by 1H NMR spectroscopy by integrating its methyl protons against the methyl protons of DMF.
1 H NMR (D2O, 300 MHz, 10 µL DMF): δ 3.22 (s; MeOH), 2.89 and 2.74 (both s, DMF).
5.5.3. Hydrogenation/Hydrosilylation of Carbon Dioxide
The above procedure was followed, but with the addition of appropriate quantity of Et3SiH, as a reaction component. To quantify MeOH and Et3SiOMe separately, samples were taken in a glove box and analyzed by
1 H NMR in dry THF-d8 using dry DMF as internal standard.
1 H NMR (THF-d8, 300 MHz, 10 µL DMF): δ 3.44 (s; MeOSiEt3), 3.29 (s; MeOH); 2.89 and 2.78 (both s,
DMF).
5.5.4. Hydrosilylation of Carbon Dioxide
A similar procedure as in 5.5.3 was adopted, but no H2 was charged. The sampling was done in glove box, analyzed by 1H NMR spectroscopy in dry THF using dry mesitylene as internal standard.
1 H NMR (THF-d8, 300 MHz, mesitylene): 3.44 (s, MeOSiEt3), 8.08 (H(CO)SiEt3), 2.24 (s, mesitylene); 2.91 and 2.78 (s, both DMF).
5.5. References
1. IPCC Fourth Assessment Report: Climate Change 2007: Synthesis Report; Chapter 2.2. 2. Annual Energy Review 2011; U.S. Energy Information Administration: Washington, DC, 2012;Table 11.1, pp 302-303. 3. USGS World Petroleum Assessment 2000 and US DOE IEA 1999, World Energy Overview. 4. N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729. 5. a) K. Ushikoshi, K. Mori, T. Watanabe, M. Takeuchi, M. Saito, Stud. Surf. Sci. Catal. 1998, 114, 357; b) M. Saito, Catal. Surv. Jpn. 1998, 175; c) L. C. Grabow, M. Mavrikakis, ACS Catal. 2011, 1,365. 6. P. W. N. M. van Leeuwen, in Homogeneous Catalysis; Understanding the art ; Kluwer Academic Publisher; Dordrecht, The Netherlands. 7. R. J. Wijngaarden, K. R. Westerterp, A. Kronberg, in Industrial catalysis: optimizing catalysts and processes; Wiley-VCH: Weinheim, 2009. 8. a) I. Karamé, In Hydrogenation; Ch. 10: InTech: Rijeka, Croatia, 2012; b) P. G. Jessop, F. Joó, C.-C. Tai, Coord. Chem. Rev. 2004, 248, 2425-2442; c). W. Wang, S. Wang, X. Ma, J. Gong, Chem. Soc. Rev. 2011, 40, 3703-3727. 9. E. Balaraman, C. Gunanathan, J. Zhang, L. J. W. Shimon, D. Milstein. Nat. Chem. 2011, 3, 609-614.
111 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
10. a) S. Chakraborty, J. Zhang, J. A. Krause, H. Guan, J. Am. Chem. Soc. 2010, 132, 8872; b) S. N. Riduan, Y. Zhang, J. Y. Ying, Angew. Chem. 2009, 121, 3372-3375; c) M.-A. Courtemanche, M.-A. Légaré, L. Maron, F.-G. Fontaine, J. Am. Chem. Soc. 2013, 135, 9326-9329 11. C. A. Huff, M. S. Sanford, J. Am. Chem. Soc. 2011, 133, 18122-18125. 12. S. Wesselbaum, T. vom Stein, J. Klankermayer, W. Leitner, Angew. Chem., Int. Ed. 2012, 51, 7499- 7502. 13. a) Y. Jiang, B. Schirmer, O. Blacque, T. Fox, S. Grimme, H. Berke, J. Am. Chem. Soc. 2013, 135, 4088-4102; b) Y. Jiang, J. Hess, T. Fox, H. Berke, H. J. Am. Chem. Soc. 2010, 132, 18233-18247. 14. Y. Jiang, O. Blacque, T. Fox, H. Berke, J. Am. Chem. Soc. 2013, 135, 7751-7760. 15. a) R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 2134; b) R. R. Schrock, J. A. Osborn, J. Am. Chem. Soc. 1976, 98, 4450; c) R. H. Crabtree, A. Gautier, G. Giordano, and T. Khan, J. Organometal. Chem. 1977, 141, 113; d) H. Berke, P. Burger, Comments Inorg.Chem. 1994, 16, 279- 312; e) H. Jacobsen, H. Berke, in Recent Advances in Hydride Chemistry; (Ed.: R. Poli), Elsevier: Amsterdam, Holland, 2001; pp 89-116; f) A. Choualeb, E. Maccaroni, O. Blacque, H. W. Schmalle, H. Berke, Organometallics 2008, 27, 3474-3481.
112 Chapter 6 Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Formates Catalyzed by Rhenium Complexes
Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Formates Catalyzed by Rhenium Complexes
6.1. Introduction
The reduction of carbonyl compounds: aldehydes, ketones, carboxylic acids and its
derivatives like esters and anhydrides to their corresponding alcohols is one of the
fundamental reactions in synthetic organic chemistry.1 Alcohols are important building blocks of intermediates and fine chemicals, pharmaceuticals and agrochemicals.2 Though
NaBH4, LiAlH4 and other stoichiometric hydride reagents including silanes are frequently used for these transformations on a laboratoty scale, the disadvantages of very low atom economy, high cost, environmentally unfriendly or not “green” and tedious work up procedures leading to large amount of waste, limit their use in industrial processes.3
Heterogeneous catalytic systems such as Pd/C and Pt/C are often used for the hydrogenation
of these carbonyl compounds, but the requirement of harsh reaction conditions, intolerance to
functional groups are the major disadvantages of these systems.2 To overcome these, there is a substantial interest in developing suitable homogeneous catalytic processes.
A number of homogeneous catalytic systems are reported for the hydrogenation of aldehydes and ketones and are often based on the platinum group metals Ir, Rh and Ru and
4 Pd. The complex [RuCl2(CO)2(PPh3)2] could hydrogenate a series of aldehydes at 15 bar H2 with TONs of up to 56000 for benzaldehyde at 180 °C and 59400 for 2-methylpentanal at
160 °C with TOFs of 4000 h-1 and 4950 h-1, respectively. These are amongst the highest
113 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
TONs reported for aldehyde hydrogenations.5 Recently, Milstein and co-workers developed efficient Fe based catalytic systems capable of hydrogenating a variety of and ketones.6 Quite
recently, Beller and co-workers have also demonstrated an effective Fe based catalytic
system capable of hydrogenating aldehydes and ketones.2
The hydrogenation of esters, bicarbonates and carbon dioxide is a difficult task.
However, sufficient attention has been paid to these transformations in the recent past.7,8,9
Though efficient methods for ester hydrogenations to the corresponding alcohols7 were
achieved, the reported hydrogenations of bicarbonates or carbonates could be taken to only to
the level of formates and not alcohols.8 The hydrogenation of carbon dioxide to formate level
has been extensively studied, but it is dominated by Ru systems.8e,f,9
The thermodynamics of the hydrogenation of CO2 to formic acid is limited by a
negative value of entropy (eq. 1).9a Addition of a base to this reaction can improve the
enthalpy, while dissolution of the gases would improve the entropy (eqs. 2-3).
CO2(g) + H2(g) → H(CO)OH(l) (eq. 1)
∆Go = 32.9 kJ/mol; ∆Ho = -31.2 kJ/mol; ∆So = -215 J/(mol K)
- + CO2(g) + H2(g) + NH3 (aq) → H(CO)O (aq) + NH4 (aq) (eq. 2)
∆Go = -9.5 kJ/mol; ∆Ho = -84.3 kJ/mol; ∆So = -250 J/(mol K)
- + CO2(aq) + H2(aq) + NH3 (aq) → H(CO)O (aq) + NH4 (aq) (eq. 3)
∆Go = -35.4 kJ/mol; ∆Ho = -59.8 kJ/mol; ∆So = -81 J/(mol K)
6.2. Results and Discussion
6.2.1. Hydrogenation of Aldehydes Catalyzed by IIIA
We have tested complex IIIA for the hydrogenation of benzaldehyde under a pressure of 50
-1 bar H2 at 140 °C in toluene. With 0.02 mol% of IIIA, a TOF of 200 h was observed in
completing the reaction in 24 h with 96% yield of benzyl alcohol in 99% conversion (Table
114 Chapter 6 Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Formates Catalyzed by Rhenium Complexes
6.1, entry 1). Via a Claisen-Tishchenko reaction 3% of benzyl benzoate was also obtained.
However, addition of 0.35 mol% of Et3SiH drastically increased the rate of the reaction.
Applying 0.0035 mol% loading of IIIA, 61% of benzyl alcohol was obtained in 17 h in 71% conversion; remaining being 4% of a hydrosilylated product, benzyl triethylsilyl ether, apart from 6% of benzyl benzoate (Table 6.1, entry 2). When the reaction was carried out with 0.02 mol% loading of IIIA and 0.25 mol% of t-BuOK as co-catalyst, the reaction showed complete conversion in 2 h with 90% yield of benzyl alcohol along with 9% yield of the ester
(Table 6.1, entry 3). At this point, it is worth mentioning that t-BuOK alone is an active catalyst for the Claisen-Tischenko disproportination of benzaldehyde to benzyl benzoate which we reported recently.10 Thus, in order to suppress the formation of benzyl benzoate,
Table 6.1. Hydrogenation of benzaldehyde catalyzed by IIIAa
IIIA (0.0035-0.02 mol%) O H2 (50 bar) O With or without base Ph OH + Ph H 120-140 oC, Toluene Ph O Ph 2a 2c 1a TOF: up to 3600 h-1 TON: up to 19200 Yield: Up to 96%
Entry IIIA Additive Time TOF (h-1) TON Yield Ester Conv. (mol%) (mol%) (h) (2a, %) (2c, %) (%) 1 0.02 - 24 200 4807 96 3 99 b 2 0.0035 Et3SiH/0.35 17 1035 17600 61 6 71 3 0.02 t-BuOK/0.25 2 2250 4500 90 9 99 4 0.01 t-BuOK /0.12 4 2400 9600 96 3 99 5 0.005 t-BuOK /0.06 1 3600 3600 18 2 20 9 2133 19200 96 3 99 6 0.005 KOH/0.06 1 2967 2967 15 1 16 7 0.005 t-BuOK /0.06 1 2855 2855 14 1 15 +TBAB/8 8 0.01 t-BuOK /0.06 16 294 4700 47 3 50c aYield by GC/MS based on the consumption of benzaldehyde; b4% of hydrosilylated product. cReaction was carried out at 120 °C.
115 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
we further reduced the amount of t-BuOK to 0.06 mol% and a run of the reaction with a
loading of 0.005 mol% of IIIA furnished benzyl alcohol with a TOF of 2133 h-1 (3600 h-1 in the first h) and a TON of 19200 in 96% yield along with 3% of the ester within a reaction time of 9 h (Table 6.1, entry 5). A batch using KOH as a base, as well as one with the addition of 5 equiv. of n-Bu4NBr in addition to t-BuOK as a base was found to be somewhat less active when compared to the action using only t-BuOK as base. A run of the reaction with a 0.01 mol% loading carried out at a pressure of 20 bar at 120 °C showed a TOF of 294 h-1 with 50% conversion in 16 h giving rise to 47% yield of benzyl alcohol.
The generality of this reaction using IIIA with addition of t-BuOK (5 equiv. with respect to IIIA) was then applied to a variety of aliphatic, aromatic and heteroaromatic aldehydes and ketones (Table 6.2). With a loading of 0.005 mol% of IIIA, 4-anisaldehyde was hydrogenated quantitatively to 4-methozybenzylalcohol in 7 h with a TOF of 2857 h-1
(Table 6.2, entry 1). Surprisingly, no competing Claisen-Tishchenko reaction was observed.
Similarly, applying a loading of 0.02 mol% of IIIA, 4-chlorobenzaldehyde was hydrogenated quantitatively to 4-chlorobenzyl alcohol in 4 h with a TOF of 1250 h-1 (Table 6.2, entry 2).
The heteroaromatic aldehydes 2-thiophenecarboxaldehyde could be hydrogenated to the corresponding alcohol when a catalyst loading of 0.02 mol% was adopted giving rise to 95% yield of the corresponding alcohol with 4% of the Claisen-Tishchenko ester in 4 h (Table 6.2, entry 3). With a catalyst loading of 0.02 mol%, the aliphatic aldehydes, cyclohexanecarboxaldehyde showed a TOF of 3920 h-1 forming 98% of the corresponding alcohol along with < 2% of the ester completing it in 1.25 h (Table 6.2, entry 4). However, under these conditions, 1-hexanal showed a TOF of 4511 h-1 furnishing 90% yield of 1- hexanol remaining being the ester with almost complete conversion (Table 6.2, entry 5). The formation of the ester in comparatively high amount is anticipated to be due to the ability of
116 Chapter 6 Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Formates Catalyzed by Rhenium Complexes
Table 6.2. Hydrogenation of various aldehydes catalyzed by IIIAa
Entry Aldehyde IIIA Time TOF TON Alcohol, Conv. (%) (1) (mol%) (h) (h-1) Yield (2, %)
1 CHO 1b 0.005 7 2857 20000 > 99 > 99
MeO
2 1c 0.02 4 1250 5000 > 99 > 99
3 1d 0.02 4 1125 4500 95 > 99
4 1e 0.02 1.25 3920 4900 98 99
5 1f 0.02 1 - 4500 90 > 99
O 6 1g 0.05 24 - 219 11 -b H H
7 1h 0.05 2 960 1920 96 > 99
aYield by GC/MS based on the consumption of substrate. bParaformaldehyde was used as the substrate and yield by 1H NMR spectroscopy using DMF as an internal standard.
t-BuOK to catalyze the Claisen-Tishchenko reaction of hexanal at room temperature.10 With a loading of 0.05 mol% of IIIA, paraformaldehyde furnished methanol with a TON of 219 and a yield of 11% in 24 h (Table 6.2, entry 6). α,β-unsaturated aldehyde, trans-cinnamaldehyde was seen to give 96% yield of 3-phenylpropanol formed by hydrogenation of both the double bonds with a TOF of 960 h-1, when a loading of 0.05 mol% of IIIA was adopted (Table 6.2, entry 7).
117 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
6.2.2. Hydrogenation of Ketones Catalyzed by IIIA
Ketones were found to be much more difficult to be hydrogenated when compared to
aldehydes. With a loading of 0.05 mol% of catalyst IIIA, acetophenone could be hydrogenated quantitatively to the corresponding alcohol, rac-1-phenylethanol in 12 h (Table
6.3, entry 1). However, under these conditions, benzophenone gave only 30% yield of the product, diphenylmethanol (5b) (Table 6.3, entry 2). 4’-Fluoroacetophenone could be hydrogenated to the corresponding alcohol rac-1-(4-fluorophenyl)ethanol (5c) in 97% yield
when a catalyst loading of 0.05 mol% was adopted (Table 6.3, entry 3). However, under these
conditions, the heteroaromatic ketone, 2-acetylthiophene was found to give a yield of only
14% of the corresponding alcohol rac-1-(2-thienyl)ethanol (5d) in 12 h (Table 6.3, entry 4).
Table 6.3. Hydrogenation of various ketones catalyzed by IIIAa
Entry Ketone (4) IIIA (mol%) TOF (h-1) TON Yield (5, %)
1 4a 167 2000 > 99 0.05
2 4b 0.05 50 600 30
3 4c 0.05 162 1940 97
4 4d 0.05 23 280 14
5 4e 0.05 - < 100 < 5
aAll the reactions were run for 12 h, yield by GC/MS based on the consumption of substrate, conversion and yield are same for entries 1-4 and for entry 5, 15% of aldol products were observed.
118 Chapter 6 Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Formates Catalyzed by Rhenium Complexes
Under these conditions, attempts to hydrogenate aliphatic ketones, like for instance cyclohexanone led to the formation of < 5% of cyclohexanol in 12 h, instead aldol products were found to dominate which presumably points to a catalysis of t-BuOK (Table 6.3, entry
5).
6.2.3. Hydrogenation of Esters and Bicarbonates to Alcohols Catalyzed by IIIA
Table 6.4. Hydrogenation of esters to alcohols and sodium bicarbonate to sodium formate or alcohols catalyzed by IIIAa
Entry Substrate IIIA Co-catalyst Solvent Product TON Yield (%) (mol%)
1 0.02 n-Bu4NBr THF PhCH2OH 500 100
THF 56 2 0.5 - CH OH 112 3
3 0.5 n-Bu4NBr EtOH CH3OH 161 81
4 0.2 - MeOH H(CO)ONa 28 5.6
5 0.2 - D2O/THF H(CO)ONa 68 13.6
6 0.5 n-Bu NBr EtOH CH3OH 20 10 4 H(CO)ONa 24 12 a All reactions were carried out under 50 bar of H2 at 140 °C run for 15 h. For entry 1, yield by GC/Ms based on the consumption of benzyl benzoate and for entries 2-6, yield by 1H NMR spectroscopy using DMF, THF or dioxane as an internal standard.
119 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Then, we tested the activity of complex IIIA in the hydrogenation of esters at pressure of 50 bar and at a temperature of 140 °C in THF. With a loading of 0.2 mol% of IIIA along with 1
mol% of n-Bu4NBr, the hydrogenation of benzyl benzoate was effected showing a TOF of >
500 h-1 giving rise to a quantitative yield of benzyl alcohol in < 1 h (Table 6.4, entry 5).
However, without the addition of base, a TON of 112 was achieved in 18 h for the hydrogenation of methyl formate to methanol when a catalyst loading of 0.5 mol% was adopted, as quantified by 1H NMR using DMF as internal standard (Table 6.4, entry 2). The carbonic acid ester, dimethylcarbonate furnished methanol in TON of 161 when run for 18 h upon hydrogenation with a loading of 0.5 mol% of IIIA along with 5 equiv. of n-Bu4NBr
(Table 6.4, entry 3).
Then, we applied IIIA as a catalyst in the hydrogenation of sodium bicarbonate under the above conditions. Thus, with a loading of 0.5 mol% of IIIA along with 5 equiv. of n-
Bu4NBr, methanol was produced with a TON of 20 and sodium formate with a TON of 24
(Table 6.4, entry 6). It is worth mentioning at this stage that the hydrogenation of carbonates or bicarbonates to methanol is unique finding and has not yet been reported in the literature.
6.2.4. Hydrogenation of Carbon Dioxide and Bicarbonates to Formates Catalyzed by IIIA
We then applied catalyst IIIA for the hydrogenation of carbon dioxide with pCO2 : pH2 = 20 : 40 at a temperature of 100 °C in the presence of 2,2,6,6-tetramethylpiperidine
(TMP) or sodium bicarbonate (Table 6.5). TMP or the bicarbonate is anticipated to function as a base and in the absence of any other additive, reaction showed a TON of 7 when a loading of 0.2 mol% was adopted in THF. With a loading of 0.2 mol% of IIIA and TMP as a base along with 5 equiv. of t-BuOK in THF gave rise to formate with a TON of 38 whereas reaction without t-BuOK gave a TON of 31 (Table 6.5, entries 2 and 3). It is worth mentioning that methanol could not be observed in these reactions. Using NaHCO3, with a
120 Chapter 6 Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Formates Catalyzed by Rhenium Complexes
a Table 6.5. Hydrogenation of CO2 to formates catalyzed by IIIA
Entry IIIA (mol%) Base Solvent TON Yield (%)
1 0.2 TMP THF 7 3.5
b 2 0.2 TMP THF-d8 38 19
3 0.2 TMP THF 31 15.5
4 0.1 NaHCO3 MeOH 605 60.5 aCatalyst loading and yield with respect to base; yield by 1H NMR spectroscopy using
DMF as internal standard. b5 equiv. of t-BuOK w. r. to IIIA was added.
loading of 0.1 mol% of IIIA along with 5 equiv. of n-Bu4NBr, a TON of 605 could be achieved when the reaction was carried out in methanol (Table 6.5, entry 4).
6.2.5. Mechanistic Aspects
Conclusive separate experiments to understand the mechanism of the hydrogenation of either aldehydes, ketones, esters or bicarbonates could not be carried out. However, a mechanism was expected to operate analogous to the one described for the imine hydrogenations using
IIIA which is also operative for hydrogenations of aldehydes or ketones without the presence of a base (Chapter 4). The accelerating influence of the addition of bromide should however be the same as described in Scheme 4.2 of Chapter 4. It is worth noting that the reaction without a base was much slower in rate when compared to those with the addition of a base.
Sampling of the hydrogenation reaction of acetopheneone and 4ʹ-fluoroacetopheneone
121 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
P O = P
Scheme 6.1. Formation of active species XXVIIA by the reaction of IIIA with t-BuOK and alcohol.
catalyzed by IIIA in the presence of t-BuOK showed only 6% and 2% conversions, respectively, in the first one hour whereas both these reactions were completed in 12 h suggests that the mechanism involving a bromide dissociation as described for the hydrogenation of imines (Chapter 4) is presumably operative at least for initial few cycles to form alcohols. However the complex IIIA showed no reaction with t-BuOK, but reacted in the presence of 2-propanol to induce catalytic transfer hydrogenations of ketones and imines
(Chapter 7). A mechanism involving a rhenium dihydride (XXVIIA, generated by the
reaction of potassium isopropoxide with IIIA) as the active species is suggested for this
transfer hydrogenation reactions. Thus, in the case of hydrogenation reactions of aldehydes
and ketones, the corresponding potassium alkoxides formed in the course of the
hydrogenation reaction is expected to generate rhenium dihydide species XXVIIA as
proposed for the hydrogenation/hydrosilylation of carbon dioxide in Chapter 5, as well as the
transfer hydrogenation reactions of ketones and imines in Chapter 7 (Scheme 6.1). Thus, in
the hydrogenation of aldehydes and ketones, the observed increase in the reaction rates after a
122 Chapter 6 Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Formates Catalyzed by Rhenium Complexes
OH NO O P H P R R' O Re R R' O = P P H NO NO P H XXVIIA P H O Re O Re H P O P O H H XXXIA R R' XXXIVA R R'
H NO NO 2 P H P H XXXIIA O Re O Re P O P O HH XXXIIIA R R' R R'
Scheme 6.2. Proposed catalytic cycle for the hydrogenation of aldehydes and ketones catalyzed by IIIA/t- BuOK system.
P O = P
Scheme 6.3. Proposed mechanism for the hydrogenation of esters catalyzed by IIIA.
123 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
certain period of time is thought to be a result of the formation of the corresponding
potassium alkoxide in the reaction medium.
Coordination of the substrate to the active species XXVIIA followed by insertion of it into the rhenium hydrides bond trans to NO ligand would generate a coordinatively
unsaturated 16e species XXXIIA (Scheme 6.2). Coordination of H2 followed by oxidative addition to XXXIIA would generate the rhenium(III) complex XXXIVA. This would
reductively eliminate the alcohol and regenerate the active species XXVIIA.
The mechanism for the hydrogenation of esters is assumed to be analogoues to the
one discussed for the hydrogenation of imines (Scheme 6.3). The ester would naturally form
first an aldehyde and an alcohol. The aldehyde would enter in to the catalytic cycle there by
hydrogenating it to alcohol.
6.3. Conclusion
Highly efficient homogeneous hydrogenations of aldehydes and ketones were realized using
nitrosyl diphosphine rhenium complexes in the presence or absence of a base. The presented
rhenium system represents one of the most active systems hydrogenation of aldehydes
compared to those reported in the literature. Claisen-Tishchenko disproportionations of
aldehydes to esters catalyzed by rhenium hydride species as well as t-BuOK were seen to be
side reactions in some cases. This could be minimized by reducing the amount of t-BuOK.
These esters formed as side products can be assumed to be hydrogenated under the given
catalytic conditions. Efficient hydrogenations of esters including that of methyl formate to
methanol could be also achieved. Both carbon dioxide and sodium bicarbonate could be
hydrogenated to the formate level and when controlling the reaction conditions these
furnished methanol as the hydrogenation product. The reaction yielding methanol by the
hydrogenation of carbon dioxide was already discussed in Chapter 5. Finally, a plausible
124 Chapter 6 Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Formates Catalyzed by Rhenium Complexes
mechanism for the hydrogenations of aldehydes and ketones, as well as of esters could be established.
6.4. Experimental Section
All manipulations of addition of reaction components and samplings were done in a glove box filled with dry
N2. All the reagents are purchased from either Aldrich or ABCR chemical company and used without further purification.
Typical Procedure for the Hydrogenation of Aldehydes and Ketones
Catalyst IIIA (0.005 g, 0.00494 mmol), benzaldehyde (10.476 g, 98.81 mmol), t-BuOK (0.0067 g, 0.059 mmol) was taken in a 50 mL stainless steel autoclave and toluene (10 mL) was added to it. The vessel was closed and connected to a Büchi pressflow gas controller machine. The gas line was evacuated thrice and the line was charged with approx. 3 bar of H2. The vessel was opened and it was evacuated carefully (thrice, not allowing pressure to go below 0 bar) to remove nitrogen. It was then charged with H2 (50 bar) and kept in an oil bath maintained at 140 °C. The consumption of the gas is measured from the graph, from which the conversion of benzaldehyde could be calculated (For reactions with a low volume of substrate, the vessel was charged with 50 bar H2 and kept in an oil bath maintained at 140 °C). After appropriate time, the vessel was cooled to room temperature, H2 was slowly released in a fume hood. The sample was taken, diluted with dichloromethane and analyzed by GC/MS (CP-3800 Saturn 2000MS/MS spectrometer, Column: Brechbuhler, ZB-5ms, 30m x
0.25mm x 0.25µm). The yield of the benzyl alcohol was calculated based on the consumption of benzaldehyde.
Benzaldehyde (1a): 3.65 min (m/z = 106); Benzyl alcohol (2a): 4.19 min (m/z = 108); Benzyl benzoate (3a):
9.61 min (m/z = 112).
GC/MS Data for other compounds (compound: retention time (mass peak)): 1b: 6.06 min (m/z = 136); 2b: 6.22 min (m/z = 138); 1c: 5.00 min (m/z: 140); 2c: 5.86 min (m/z = 142); 1d: 3.93 min (m/z = 112); 2d: 4.12 min
(m/z = 114); 3d: 8.32 min (224); 1e: 3.45 min (m/z = 112); 2e: 3.93 min (m/z = 114); 3e: 9.20 min (m/z = 224);
1f: 1.81 min (m/z: 100); 2f: 2.82 min (m/z = 102); 1g: 4.19 min (m/z = 132); 2g: 6.42 min (m/z = 136); 3f: 6.83 min (m/z = 200); 4a: 4.44 min (m/z = 120); 5a: 4.37 min (m/z: 122); 4b: 35.51 min (m/z = 182); 5b: 36.00 min
(m/z = 184); 4c: 4.34 min (m/z = 138); 5c: 4.47 min (m/z = 140); 4d: 4.37 min (m/z = 126); 5d: 4.67 min (m/z =
128); 4e: 3.10 min (m/z = 98); 5e: 3.02 min (m/z = 100).
125 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Typical Procedure for the Hydrogenation of Esters and Sodium Bicarbonate
Catalyst IIIA (0.005 g, 0.00494 mmol), benzyl benzoate (0.524 g, 2.47 mmol), n-Bu4NBr (0.008 g, 0.0247 mmol ) was taken in a stainless steel autoclave and THF (1 mL) was added to it. The vessel was then charged with H2 (50 bar) and kept in an oil bath maintained at 140 °C. After appropriate time, the vessel was cooled to room temperature, H2 was slowly released in a fume hood. The sample was analyzed by GC/MS and yield was calculated based on the consumption of benzyl benzoate.
For reactions in Table 6.4, entries 2, 3 and 6, after the reaction, 10 µL of DMF was added as internal standard and the yield of methanol was calculated by 1H NMR spectroscopy based the methyl peaks of DMF. For entry 6, the yield of sodium formate was determined based on the formyl H proton of DMF.
1 H NMR (D2O, 300 MHz, 10 µL DMF): δ 3.22 (s; methanol), 2.89 and 2.74 (both s, DMF).
For reaction in Table 6.4, entry 4, the reactions mass was evaporated to dryness. The mixture is then weighed.
From this, 20 mg was taken in an NMR tube. D2O was added along with 10 µL of THF. The formate content was analyzed by 1H NMR spectroscopy.
1 H NMR (D2O, 300 MHz, 10 µL THF): δ 8.41 (s; sodium formate), 3.73 and 1.87 (both unresolved s, THF).
For reaction in Table 6.4, entry 4, the reactions mass was evaporated to dryness. D2O was added to the whole mass along with 20 µL of dioxane. The formate content was analyzed by 1H NMR spectroscopy.
1 H NMR (D2O, 300 MHz, 20 µL dioxane for entry 4): δ 8.46 (s; sodium formate), 3.74 (s, dioxane).
Typical Procedure for the Hydrogenation of Carbon Dioxie to Formate Salts
Catalyst IIIA (0.005 g, 0.00494 mmol) and n-Bu4NBr (0.008 g, 0.0247 mmol ) was taken in a stainless steel autoclave and THF (1 mL) was added to it. The vessel was then charged with CO2 (20 bar; note that a pressure reduction was observed and reached ~ 18 bar) followed by H2 (40 bar; making a total pressure of ~ 58 bar) and it was kept in an oil bath maintained at 140 °C for 15 h with stirring. The vessel was cooled to room temperature, H2 was slowly released in a fume hood. DMF (10 µL for entries 1-3 and 20 µL for entry 4) was added to the whole mass. The sample was analyzed by 1H NMR spectroscopy. The formate content was determined based on the formyl H peak of DMF.
1 H NMR (D2O, 300 MHz, 10 or 20 µL DMF): δ 8.31 (s; sodium formate), 7.81 (s, DMF).
6.5. References 1. H.-U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner, M. Studer, Adv. Synth. Catal. 2003, 345, 103-151. 2. S. Fleischer, S. Zhou, K. Junge, M. Beller. Angew. Chem. Int. Ed. 2013, 52, 5120-5124.
126 Chapter 6 Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Formates Catalyzed by Rhenium Complexes
3. M. L. Clarke, G. J. Roff, in Handbook of Homogeneous Hydrogenation, (Eds.: J. G. de Vries, C. J. Elsevier), Wiley-VCH: Weinheim, 2007; pp 413-437. 4. For reviews on this topic, see: a) F. Spindler, H.-U. Blaser in Transition Metals for Organic Synthesis, Vol. 2 (Eds.: M. Beller, C. Bolm), Wiley-VCH, Weinheim, 2004, p. 113; b) H.-U. Blaser, F. Spindler in Handbook of Homogeneous Hydrogenation, Vol. 3 (Eds.: J. G. de Vires, C. J. Elsevier), Wiley- VCH, Weinheim, 2007, p. 1193; c) M. J. Palmer, M.Wills, Tetrahedron: Asymmetry 1999, 10, 2045- 2061; d)W. Tang, X. Zhang, Chem. Rev. 2003, 103, 3029-3069; e) S.-L. You, Chem. Asian J. 2007, 2, 820-827; f) R. H. Morris, Chem. Soc. Rev. 2009, 38, 2282-2291; g) T. C. Nugent, M. EI-Shazly, Adv. Synth. Catal. 2010, 352, 753-819; h) M. Rüping, E. Sugiono, F. R. Schoepke, Synlett 2010, 852-865; i) N. Fleury-Brégeot, V. de La Fuente, S. Castillon, C. Claver, ChemCatChem 2010, 2, 1346- 1371; j) J.- H. Xie, S.-F. Zhu, Q.-L. Zhou, Chem. Rev. 2011, 111, 1713 -1760. 5. W. Strohmeier, L. Weigelt, J. Organomet. Chem. 1978, 145, 189. 6. a) R. Langer, G. Leitus, Y. Ben-David, D. Milstein, Angew. Chem. 2011, 123, 2168-2172; Angew. Chem. Int. Ed. 2011, 50, 2120-2124; b) R. Langer, M. A. Iron, L. Konstantinowsky, Y. DiskinPosner, G. Leitus, Y. Ben-David, D. Milstein, Chem. Eur. J. 2012, 18, 7196 -7209. 7. a) M. L. Clarke, Catal. Sci. Technol. 2012, 2, 2418-2423; W. N. O. Wylie, R. H. Morris, ACS Catal. 2013, 3, 32-40; b) E. Balaraman1, C. Gunanathan1, J. Zhang, L. J. W. Shimon, D. Milstein1, Nat. Chem. 2011, 3, 609-614; c) M. L. Clarke, M. B. Díaz-Valenzuela, A. M. Z. Slawin, Organometallics, 2007, 26, 16-19. 8. a) C. Federsel, A. Boddien, R. Jackstell, R. Jennerjahn, P. J. Dyson, R. Scopelliti, G. Laurenczy, M. Beller, Angew. Chem. Int. Ed. 2010, 49, 1-6; b) C. Federsel, R. Jackstell, A. Boddien, G. Laurenczy, M. Beller, ChemSusChem 2010, 3, 1048-1050; c) A. Boddien, F. Grtner, C. Federsel, P. Sponholz, D. Mellmann, R.Jackstell, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2011, 50, 6411-6414; d) C. Ziebart, C. Federsel, P.Anbarasan, R. Jackstell, W. Baumann, A. Spannenberg, M. Beller, J. Am. Chem. Soc. 2012, 134, 20701-20704. e) J. Elek, L. Nádasdi, G. Papp, G. Laurenczy, F. Joó, Appl.Catal. A: Gen.2003, 255, 59-67; f) Á. Kathó, Z. Opre, G. Laurenczy, F. Joó, J. Mol. Catal. A: Chem. 2003, 204– 205, 143-148. 9. a) P. G. Jessop, T. Ikariya, R. Noyori, Chem. Rev. 1995, 95, 259-272; b) P. G. Jessop, F. Joó, C.-C. Tai, Coord. Chem. Rev. 2004, 248, 2425–2442; c) P. G. Jessop, T. Ikariya, R. Noyori, Nature, 1994, 368, 231-233; d) P. G. Jessop, Y. Hsiao, T. Ikariya, R. Noyori J. Am. Chem. Soc. 1994,116, 8851-8852; e) R. Tanaka, M. Yamashita, K. Nozaki, J. Am. Chem. Soc. 2009, 131, 14168-14169; f) Y. Jiang, B. Schirmer, O. Blacque, T. Fox, S. Grimme, H. Berke, J. Am. Chem. Soc. 2013, 135, 4088-4102. 10. K. Rajesh, H, Berke, Adv. Synth. Catal. 2013, 355, 901-906.
127 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
7.1. Introduction
The developments in organometallic chemistry have widely strengthened the concept of
homogeneous catalysis and today it can be applied to most of the chemical transformations in
industry to serve an increasing demand of the world. The growing concern on environmental
conservation focuses chemistry-wise mainly on catalytic strategies. Molecular catalysts offer
improved selectivity, increased activity, and allow operationally lower temperatures. Ester
synthesis and hydrogenation of organic substrates are among the many fundamental
transformations of fine chemical industry. The atom economic Claisen-Tishchenko
disproportionation of aldehydes to the corresponding carboxylic esters (Scheme 7.1)1 has
acquired wide attention due to their application in food, polymer, dye and perfume industry.2
Traditional catalysts for this reaction include mainly sodium[3a-b] and particularly aluminium
3c-g 3h 3i 3j,k 3l 3b,m-n alkoxides. boric acid, i-Bu2AlH, alkaline earth metal amides, LiBr/Et3N, NaH,
Grignard reagents in combination with thiolates,3o selenide ions3p were also reported as
catalysts derived from main group elements. Very recently, we reported alkali metal tert-
butoxides, hydrides and bis(trimethylsilyl)amides as efficient catalysts for this reaction, in
which the former two species were realized to be the best known catalysts reported among
the class of catalysts derived from main group elements.4 N-heterocyclic carbenes,5 transition
metal complexes based on Fe,6a-b Ru,6c-h Rh,6i-k Os,6l Ir,6m-n Ni,6o-p Zr,6q Hf6q and lanthanide
128 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
complexes,7a-g particularly lanthanide amides7b-e and organoactinide complexes7h-i have also been employed for this reaction. Recently, this disproportionation reaction between two different selected aldehydes could be accomplished in good selectivities using a metal complex of Ni.1f,6p
Scheme 7.1. Claisen-Tishchenko Reaction
7.2. Claisen-Tishchenko Reaction of Aldehydes Catalysed by IIIA
7.2.1. Results and Discussion
When we started to study the activity of complexes IIIA, IIA, VIIA and VIIB in combination with Et3SiH for the hydrogenation of carbonyl compounds, we came across the
formation of benzyl benzoate during the hydrogenation of benzaldehyde (chapter 6). This led
us to investigate the scope of this complex as a catalyst in Claisen-Tishchenko type
disproportionations. This reaction using the rhenium complex IIIA or any of the tested
silanes could not give the product. Only the combination of both established catalytic
transformations.
The feasibility of the catalytic Claisen-Tishchenko reaction has been studied using
benzaldehyde as model substrate with 0.05 mol% of complex IIIA along with variable quantities of different organosilanes as co-catalysts in THF at 110 °C (Table 7.1). None of the tested silanes could give complete conversion due to concomitant hydrosilylations in most cases. Also, sterically demanding silanes either did not show any reaction or gave lower yields (Table 7.1, entries 2, 3 and 5). In terms of activity, diphenyl silane (Ph2SiH2) was found to be the best choice; using 2.5 mol% showed a TOF of 3280 h-1 and 82% GC yield of benzyl benzoate within 0.5 h in 85% conversion (Table 7.1, entry 7). When only 1.5 mol% of
129 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Table 7.1. Claisen-Tishchenko reaction of benzaldehyde using IIIA/various silyl hydride systema
Entry Silane/Mol% TOF (h-1) Yield (%) Conv. (%)
1 Et3SiH/2.5 2648 67 70
2 i-Pr3SiH/2.5 - - -
3 PhMe2SiH/2.5 118 30 35
4 (MeO)3SiH/2.5 2360 59 63
5 Ph3SiH/2.5 - - -
6 PhSiH3/2.5 2520 63 66
7 Ph2SiH2/2.5 3280 82 85
8 Ph2SiH2/1.5 3080 77 79
b 9 Ph2SiH2/(1.5+5) - 88 94
10 Ph2SiH2/8 3360 84 92
c 11 Ph2SiH2/2.5 2453 61 65
d 12 Ph2SiH2/5 1497 75 87
aYield by GC/MS based on the consumption of aldehyde. bReaction run for another 1 h after adding 5 mol% c d more of Ph2SiH2. Reaction was carried out at 90 °C. 0.1 mol% of IIIA was used.
-1 Ph2SiH2 was used, a TOF of 3080 h was observed with a conversion of 79% giving rise to
77% yield of benzyl benzoate in 0.5 h (Table 7.1, entry 8). Additional 5 mol% of Ph2SiH2 was added to this reaction mixture to further advance it giving rise to a yield of 88% of benzyl benzoate with 94% conversion in another 1 h (Table 7.1, entry 9). However, addition of a higher quantity of Ph2SiH2 (8 mol%) at the beginning itself gave 84% yield in 92%
130 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
conversion within 0.5 h (Table 7.1, entry 10). Lowering the temperature to 90 °C showed a
TOF of 2453 h-1 with a yield of 61% of benzyl benzoate in 65% conversion (Table 7.1, entry
11). Reaction was then carried out with a double loading of IIIA (0.1 mol%) along with 5
-1 mol% of Ph2SiH2. This gave a TOF of 1497 h with a yield of 75% of the product in 87% conversion (Table 7.1, entry 12).
The reaction of IIIA or of the mixture of IIA and IIIA (~ 2:1 obtained after a fresh
° preparation) with excess of Et3SiH (20 equiv.) in THF-d8 at 70 C gave same mixture of
products in both cases, which were stable only in the reaction solution. NMR study revealed
the major product as a dihydride silyl rhenium complex (chapter 2). In order to overcome the
limitations of incomplete conversion and the formation of undesired side products in the
silane co-catalyzed Claisen-Tishchenko reaction of aldehydes, we thought of preparing
suitable stable hydride species using other hydride reagents. Thus, the reaction of IIIA with 5
equiv. of LiAlH4 in THF at room temperature was probed rise to immediate formation of a
mixture of rhenium hydrides analyzed in situ by 1H and 31P NMR spectroscopy. Detailed in
situ NMR studies revealed one of them to be a rhenium trihydride species XXXVA (Scheme
7.2), (1H NMR, trihydride): δ -5.58 (m, 1H), -3.20; (m, 2H,); other hydride: -9.02 (br, m).
However, attempts of isolation of these complexes in the solid state led to formation of
mixtures of various other rhenium hydride species as yet not fully characterized.
Then, this in situ prepared mixture of rhenium hydrides (obtained by the reaction of
IIIA with LiAlH4 in THF) were tested for the catalytic Claisen-Tishchenko
disproportionation reaction (Table 7.2). With a 0.1 mol% loading of IIIA along with 0.5
mol% of LiAlH4, at room temperature, though the disproportionation of benzaldehyde to
benzyl benzoate proceeded, it was found to be slow giving rise to a GC yield of < 20% in 12
h (Table 7.2, entry 1). As expected, < 1% of benzyl alcohol was also formed primarily due to
131 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
a Table 7.2. Claisen-Tishchenko reaction of aldehydes using IIIA/LiAlH4 system.
Entry Aldehyde IIIA/ Temp. TOF Time Yield Alcohol (%) (1) mol% (°C) (1st h) (h) (2, %)
1 1a 23 11 12 <20 < 1 0.1 2 80 260 10 97 < 1
3 1b 0.2 110 275 5 96 < 1
4 1c 0.2 110 495 1 99 < 1
5 1d 0.2 110 391 3.5 97 < 1
6 1e 0.2 110 150 16 90 2
7 1f 0.2 23 490 <1 98 < 1
8 1g 0.2 23 465 <1 93 b < 1
9 1h 0.2 23 435 1.5 96 < 1
10 1i 0.2 110 121 1 24 b,c < 1 aUnless mentioned, TOF and yield by GC/MS based on the consumption of aldehydes. b yield by 1H NMR spectroscopy using an internal standard; naphthalene for entry 8 and mesitylene for entry 10. cParaformaldehyde was used as the substrate.
132 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
the hydrogenation of benzaldehyde by LiAlH4.Under the same loadings, but at a temperature of 80 °C, benzyl benzoate was obtained in 97% GC yield in 10 h with a TOF of 260 h-1 for
the first hour (Table 7.2, entry 2). It is worth mentioning that similar results were obtained
when a mixture of IIA and IIIA (~ 2:1 obtained immediately after preparation) was used as
catalyst instead of only IIIA. Without the addition of any substrate, when the in situ formed
rhenium hydrides (obtained by the reaction of IIIA with LiAlH4 in THF) were heated to 80
°C, we observed the disappearance of the trihydride species where the other species
remained. Addition of 20 equiv. of benzaldehyde to this solution did not give any benzyl
benzoate at room temperature as well as at a temperature of 80 °C indicating the disappeared
rhenium trihydride species was responsible for this disproportination reaction. This strategy
was adopted for Claisen-Tishchenko disproportionation of a selection of other aromatic and
heteroaromatic aldehydes using complex IIIA as catalyst, but carried out at a still higher
loading of 0.2 mol% of IIIA and 1 mol% of LiAlH4 at a temperature of 110 °C (Table 7.2,
entries 3-6). The electron rich 4-anisaldehyde showed a TOF of 495 h-1 giving rise to 99%
yield of the desired ester within an hour (Table 7.2, entry 4). Also, the electron rich
heteroaromatic aldehyde, 2-thiophenecarboxaldehyde, gave a yield of 97% of the desired
ester in 3.5 h (Table 7.2, entry 5), whereas the electron deficient heteroaromatic aldehyde, 2-
furfuraldehyde, which is known to be difficult to undergo the Claisen-Tishchenko reaction3n,8
gave only 90% yield of the desired ester in 16 h (Table 7.2, entry 6). Then we tested aliphatic
primary and secondary aldehydes for this reaction. Unlike aromatic and heteroaromatic
aldehydes, these aliphatic aldehydes could be smoothly converted to the corresponding esters
at room temperature. The primary aldehyde, hexanal could be disproportionated with a
loading of 0.2 mol% of IIIA and 1 mol% of LiAlH4 in THF at room temperature to obtain the
corresponding ester, hexyl hexanoate, in 98% yield within 1 h showing a TOF of 490 h-1
(Table 7.2, entry 7). Under these conditions, the secondary aldehydes, isobutyraldehyde and
133 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
cyclohexanecarboxaldehyde gave yields of the corresponding esters in 93% and 96%
respectively (Table 7.2, entry 8 and 9). Claisen-Tishchenko disproportionation of
paraformaldehyde could also be carried out at a temperature of 110 °C in THF when a
catalyst loading of 0.2 mol% and a LiAlH4 loading of 1 mol% was applied leading to the desired ester, methyl formate, in 25% yield in 5 h, as analyzed and quantified by 1H NMR spectroscopy using mesitylene as internal standard (Table 7.2, entry 10).
7.2.2. Mechanistic Studies
Claisen-Tishchenko disproportionations of aldehydes are envisaged to involve the transfer of an aldehydic hydride to the aldehydic carbon of the other aldehyde as the key step. Addition of 20 equiv. of benzaldehyde at room temperature to the mixture containing rhenium trihydride species XXXVA led to the disappearence of this trihydride species with the formation of benzyl benzoate and benzyl alcohol as analyzed by 1H NMR. Benzyl alcohol was generated mainly by reduction of benzaldehyde with LiAlH4. The anionic complex
XXXVA would have underwent dissociation of a hydride ligand there by leaving LiAlH2Br2 and generating the active species XXVIIIA (Scheme 7.2). Also, as discussed, rhenium hydrides could be obtained by the reaction of IIIA with Et3SiH which gave the hydrosilylated benzyltriethylsilyl ether as a by-product of the disproportionation reaction of the benzaldehyde. Presumably due to a too large bite angle of the Sixantphos ligand, the sixantphos hydride complexes were unstable. Thus sixantphos complex analogous to the rhenium hydride complex VIE bearing comparatively smaller bite angle 1,1ʼ- bis(diphenylphosphino)ferrocene (dppf; E) ligand could not be obtained on the reaction of
IIA or IIIA with Et3SiH followed by ethylene, it underwent an ortho-metallation instead
(Chapter 2). Nevertheless, hydrogenolysis of the ortho-metallated rhenium carbon bond led to the formation of the 16 e- rhenium hydride VA, the active species for olefin hydrogenations.
134 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
The complex VIE catalyzed the disproportionation even in the absence of a co-catalyst, but
was far less active. These reactions also revealed formation of benzyl alcohol indicating that
the reaction may have proceeded via a rhenium alkoxide species. Kinetic studies using the
IIIA/LiAlH4 system on the disproportionation reaction of hexanal to hexyl hexanoate showed
a linear relationship between ln[reactant] and time peaking for a first order dependency with
respect to the substrate (Figure 7. 1). Thus, from these observations, one can conclude that
the disproportionation reaction of aldehydes by the rhenium hydride system VIE, or those formed either by the reaction of IIIA with LiAlH4 or silane would expected to pass through a rhenium alkoxide intermediate formed by the hydride transfer to aldehydes or insertion of aldehyde into the rhenium hydride bonds (Scheme 7.2).[6h] Subsequent coordination of a molecule of aldehyde to alkoxide complex followed by insertion into the rhenium alkoxide bond results in the formation of a rhenium hemiacetal species. In the case of the silane co- catalyzed reaction, the coordinated aldehyde and the coordinated silyl group could be in competitive for insertion into the rhenium alkoxide bond or for reductive elimination of the hydrosilylated ether product. The hemiacetal species formed by the insertion of aldehyde into
-0.4
-0.5
-0.6
-0.7 ln[hexanal] -0.8
-0.9
0 5000 10000 15000 20000 time (min)
Figure 7. 1. Linear plot of ln[hexanal] vs time for the Claisen-Tishchenko reaction using IIIA/LiAlH4 system indicating a first order kinetic with respect to hexanal.
135 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
P O = P
Scheme 7.2. Proposed catalytic cycle for the Claisen-Tishchenko reaction of aldehydes using IIIA/LiAlH4 as catalyst.
the rhenium alkoxide bond would eventually undergo β-hydride shift eliminating the ester and regenerating the rhenium hydride species.
7.3. Transfer Hydrogenations of Ketones and Imines
The key step involved in Claisen-Tishchenko reactions is the hydride transfer from one molecule of aldehyde to another molecule of aldehyde and subsequent coupling between the two species. Thus, one can expect that metal complexes capable of performing this
136 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
transformation can often also catalyze transfer hydrogenations of polar substrates like
aldehydes, ketones and imines (Scheme 7.3). Like catalytic hydrogenation, catalytic transfer
hydrogenation is also considered to be an environmentally friendly transformation, the latter
even much safer and convenient to handle on any scale. Most of the reported transition metal
catalyzed transfer hydrogenation reactions of ketones and imines are designed to operate
through Shvo9,10 or Noyori9,11 type secondary coordination sphere metal ligand bifunctional mechanisms with simultaneous proton and hydride transfers.12 On the other hand, primary
Scheme 7.3. General sketch of the transfer hydrogenation reaction
coordination sphere mechanisms operating through the availability of a metal hydride and a vacant site are also reported extensively on ruthenium systems particularly bearing monodentate phosphines.9,13
7.3.1. Transfer Hydrogenation of Ketones Catalyzed by IIIA
7.3.1.1. Results and Discussion
Since the in situ generated rhenium hydrides are active catalysts for the Claisen-Tishchenko
reaction, we thought these would also be active catalysts for the transfer hydrogenation of
aldehydes, ketones and imines which were in many parts of the catalytic cycles expected to
be related. In an exploratory way, transfer hydrogenations of acetophenone were carried out
testing different conditions (Table 7.3). Catalysis could not be observed with only IIIA.
Using IIIA in the presence of the silane Ph2SiH2 as co-catalyst, the reaction was found to be far less efficient even at higher temperatures when compared to the IIIA/LiAlH4 or IIIA/base
catalytic systems (Table 7.3, entry 1). With loadings of 0.2 mol% of IIIA and 1 mol% of
137 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
LiAlH4 or t-BuOK using 10 equiv. of 2-propanol as the hydrogen donor gave a conversions of only <10% in both cases when run for 12 h at room temperature (Table 7.3, entries 2 and
3). However, when these reactions were carried out at 83 °C, 83% yield of the product 1- phenylethanol was obtained within an hour in both the cases (Table 7.3, entries 4 and 5). The t-BuOK co-catalyzed reaction at a further elevated temperature of 110 °C also did not give considerable improvement (Table 7.3, entry 6). With the same loadings at 83 °C, reaction using KOH gave a little less of the product (Table 7.3, entry 7). Still higher loadings of 0.4 mol% of the catalyst and 2 mol% of t-BuOK along with 20 equiv. of 2-propanol could give a yield of 89% of the alcohol (Table 7.3, entry 8). However, under any of the above conditions and loadings, no further progress of the reaction was observed. Now, samples were taken in
0.25 h for the transfer hydrogenation of acetophenone; with loadings 0.2 mol% of IIIA and 1 mol% t-BuOK in 20 equiv. of 2-propanol, which gave 89% of the desired alcohol showing a
TOF of 1780 h-1 (Table 7.3, entry 10). Thus, the reaction with higher loadings of catalysts
(Table 7.3, entry 8) would have achieved this yield within 0.25 h and the other reactions
(Table 7.3 entries 4-7 and 9) would have achieved in 0.25 h, yields almost close to those observed in 1 h. This is further concluded from a yield of 81%, obtained when sample was taken for this reaction with loadings of 0.2 mol% IIIA and 1 mol% t-BuOK (Table 7.3, entry
4). A yield of 89% was obtained even with a loading of only 0.05 mol% of IIIA along with 1 mol% of t-BuOK with 20 equiv. of 2-propanol in 1 h for which a TOF of 6160 h-1 with 77% yield was observed in 0.25 h (Table 7.3, entry 11). When the loading was further reduced to
0.02 mol%, 46% of the alcohol was formed in 0.25 h with a TOF of 9200 h-1 giving rise to a yield of 89% of the desired alcohol in 4 h (Table 7.3, entry 12).
Then, this strategy of transfer hydrogenation was tested for various ketones keeping the t-BuOK loading as 1 mol% and 2-propanol amount as 20 equiv., but varying the loadings
138 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
Table 7.3. Transfer hydrogenation of acetophenone using 2/additive in 2-propanol.a
Entry IIIA/ Additive/mol% Donor Temp. TOF Time Yield mol% /Equiv. (°C) (h-1) (h) (4, %)
1 0.2 Ph2SiH2/1 10 100 - 1 <10
2 0.2 LiAlH4/1 10 23 - 1 <10
3 0.2 t-BuOK/1 10 23 - 1 <10
1620 0.25 81 4 0.2 t-BuOK/1 10 83 >415 1 83
5 0.2 LiAlH4/1 10 83 >415 1 83
6 0.2 t-BuOK/1 10 110 >425 1 85
7 0.2 KOH/1 10 83 >395 1 79
8 0.4 t-BuOK/2 10 83 >223 1 89
9 0.2 t-BuOK/1 5 83 1460 1 73
10 0.2 t-BuOK/1 20 83 >1780 0.25 89
11 0.05 t-BuOK/1 20 83 6160 0.25 77 1 89
12 0.02 t-BuOK/1 20 83 9200 0.25 46 4 89 aYield by GC/MS based on the consumption of acetophenone.
of IIIA. Thus, the conditions of 0.02 mol% of IIIA, the transfer hydrogenation of 4- methoxyacetophenone showed a TOF of 6191 h-1 in the first 0.25 h giving rise to 66% yield of the corresponding alcohol in 10 h (Table 7.4, entry 1). However, the conversion could not
139 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
be improved considerably even with a higher loading of 0.2 mol% of IIIA (Table 7.4, entry
2). 4-methylacetophenone under the loading of 0.2 mol% showed a TOF of 1529 h-1 giving rise to a yield of 82% of the desired alcohol in 1 h (Table 7.4, entry 3). Transfer hydrogenation of 4-chloroacetophenone with 0.05 mol% loading of IIIA showed a TOF of
6090 h-1 in the first 0.25 h giving rise to a yield of 95% of the corresponding alcohol in < 8 h
(Table 7.4, entry 4). With a loading of 0.05 mol%, 2-acetylthiophene showed a TOF of 3568 h-1 in the first 0.25 h giving rise to 66% yield of the corresponding alcohol in < 4 h (Table
7.4, entry 5). However, the yield could be improved to only 72% even with a fourfold loading of 0.2 mol% of IIIA (Table 7.4, entry 6). Under these conditions, with a loading of 0.02 mol%, the transfer hydrogenation of benzophenone showed a TOF of 3554 h-1 in the first
0.25 h giving rise to a yield of 95% of the desired alcohol in 20 h (Table 7.4, entry 7). The transfer hydrogenation of 3,3’- bis(trifluoromethyl)benzophenone was carried out with a loading of 0.2 mol% of IIIA showed a TOF of 1060 h-1 in the first 0.25 h giving rise to a
yield of > 99% yield of the desired alcohol in < 6 h (Table 7.4, entry 8). Attempting transfer
hydrogenation of aliphatic ketones, cyclohexanone was used first which with a loading of
0.02 mol% of IIIA showed a TOF of 3560 h-1 in the first 0.25 h and 99% yield of
cyclohexanol in 18 h (Table 7.4, entry 9). The α,β-unsaturated ketone, 2-cyclohexenone, with
0.2 mol% of the catalyst IIIA showed in 0.25 h the formation of 41% yield of cyclohexanol,
however 51% of the aldol product of cyclohexanone was observed with a conversion of 98%.
In order to suppress the aldol formation, t-BuOK loading was reduced to 0.2 mol%; thus with
0.05 mol% of IIIA, 26% of cyclohexanol, 19% of cyclohexanone, 15% of the aldol products
were observed in the first 0.25 h. However, further run of this reaction to 2 h gave a yield of
74% of cyclohexanol, 1% of cyclohexanone and 19% of the aldol products with 99%
conversion (Table 7.4, entry 10).
140 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
Table 7.4. Transfer hydrogenation of various ketones using IIIA/t-BuOK in 2-propanol.a
Entry Ketone IIIA (mol%) TOF (h-1) Time Yield (%) 3 (h) 4
1 0.02 6191 0.25 31
2056 1 41
328 10 66 3b 2 0.2 1254 0.25 63
324 1 65
34 10 68
3 0.2 1529 0.25 76 3c 403 1 82
4 0.05 6090 0.25 76
3d 4208 1 84
239 8 95
5 0.05 3568 0.25 45
3e 5 66
6 0.2 1309 0.25 65
4 72
7 3f 0.02 3554 0.25 18
20 95
141 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
8 0.2 1061 0.25 53 3g 100 4 >99
9 3h 0.02 3560 0.25 18
102 18 99
10b 3i 0.05 2108 0.25 26 c
370 4 74c
aYield by GC/MS based on the consumption of substrate. b0.2 mol% of t-BuOK was used. ccyclohexanol.
The failure to improve the reaction even with higher loadings of the catalyst in the
case of few of the above reactions can be anticipated to be a consequence of the
thermodynamic equilibrium attaining in those reactions.
Then the transfer hydrogenation of aldehydes was probed using 4-chlorobenzaldehyde
with loadings of 0.05 mol% of IIIA and 1 mol% of t-BuOK which gave 24% of 4-
chlorobenzylalcohol, 34% of the mixed ester isopropyl(4-chlorobenzoate), 8% of the
disproportionative ester 4-chlorobenzyl-4-chlorobenzoate and 1% of t-butyl(4- chlorobenzoate) with 67% conversion in 24 h. The Claisen-Tishchenko products were anticipated to be formed through separate catalytic cycles operating rhenium alkoxide, as well as potassium alkoxide species.
7.3.2. Transfer Hydrogenation of Imines
7.3.2.1. Results and Discussion
Pursuing the transfer hydrogenations of imines, the reactions could be brought to near completion for most of the tested imines, but a comparatively higher loading of 0.5 mol% of
IIIA had to be adopted (Table 7.5). Thus, at 83 °C with 20 equiv. of 2-propanol, the transfer
142 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
Table 7.5. Transfer hydrogenation of various imines using IIIA/t-BuOK in 2-propanol.a
Entry Imine IIIA/ TOF (h-1) Time Yield (%) 5 mol% (h) 6
1 5a 0.5 136 3 95
2 5b 0.5 127 3 89 N
O
3 5c 0.5 129 3 98
4 5d 0.5 170 2 99
5 5e 0.5 167 2 95
6 5f 1 60 5 48
aYield by GC/MS based on the consumption of imine.
hydrogenation of N-benzylidineaniline showed a TOF of 136 h-1 in the first hour giving rise to a yield of 95% of the desired product N-benzylaniline in 3 h (Table 7.5, entry 1). This strategy was also adopted for transfer hydrogenations of other imines. Keeping the other
143 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
parameters unchanged, but increasing the loading of catalyst IIIA to 1 mol%, N-
benzylideneisobutylamine could be converted to the desired product N-benzylisobutylamine
with 48% yield in 5 h (Table 7.5, entry 6).
7.3.3. Mechanistic Studies
In order to establish the mechanism of these transfer hydrogenations reaction, labelings as
well as kinetic experiments were conducted using benzophenone as a substrate.[9a] As mentioned, the reaction did not give any product when base was not added. IIIA, insoluble in
benzene-d6 at room temperature did not show any reaction when 3 equiv. of t-BuOK was added, but addition of 5 equiv. of 2-propanol to this mixture showed immediate reaction at this temperature evolving resonances in the 1H and 31P NMR spectra and a by-product identified as acetone. So, the base potassium isopropoxide (t-BuOK in 2-propanol) was assumed to have abstracted at least one of the bromides from the rhenium centre of IIIA to generate a rhenium isopropoxide species, which had undergone β-hydride abstraction to generate the rhenium hydride species eliminating acetone, as reported for the well known
Figure 7. 2. TOF (after 10 min) vs concentration 2-propanol on the transfer hydrogenation of benzophenone using IIIA/t-BuOK system.
144 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
hydrogenation of ketones and imines (Scheme 7.4).9 Evidence for this type of mechanism
could be provided from the ability of the rhenium monohydride complex VIE alone, which though far much less efficient showed the formation of the desired alcohol. Addition of 1 mol% of t-BuOK led to the increase in TOF of this transfer hydrogenation reaction, which could be explained as a consequence of the abstraction of the bromide to generate a rhenium dihydride species.11b,c So it is assumed that both the bromides in complex IIIA are substituted by the base to form a rhenium dihydride species.
Me Me NO Si P P H [Re] O = = O Re NO P O P H P PPh PPh O Re 2 A 2 P H XXVIIA
Scheme 7.4. Proposed catalytic cycle for the transfer hydrogenation of ketones using IIIA/t-BuOK/2-propanol system.
Scheme 7.5. Reaction of benzophenone with (CH3)2CH(OD).
145 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
The catalytic transfer hydrogenation reaction of benzophenone using (CH3)2CH(OD) gave Ph2(CH)OD (Scheme 7.5) with the content of a label > 95% indicating a reaction in
which the hydrides on rhenium are derived from the methyne carbon atoms of 2-propanol
ruling out any possibility of an oxidative addition of 2-propanol to the rhenium centre as
11b,c proposed for Ru(PPh3)3Cl2-base co-catalyzed transfer hydrogenations. Coordination of the
substrate to the rhenium dihydrides species followed by insertion of it into the Re-H bond
would generate a rhenium alkoxide species. It can be concluded that the rhenium alkoxide
species is protonated by 2-propanol to form the desired alcohol, there by regenerating the
rhenium isopropoxide species. Carrying out the reaction with 0.2 mol% of IIIA and 1 mol% of t-BuOK, but with 10 equiv. of (CH3)2CH(OH), (CH3)2CH(OD) and (CD3)2CD(OD), a
kinetic isotopic effects KHH/KHD/KDD could be established in a ratio of 1:1.35:2.0, respectively at 83 °C when run for 15 min. This revealed that either the β-hydride abstraction or the insertion of the substrate into the Re-H bond (hydride transfer) would be the rate limiting step. At lower concentrations (2-6 equiv.) of 2-propanol, the conversion was found to show a linear dependency which suggested the former to be the rate determining step (Figure
Figure 7.3. TOF (rate) vs time for the transfer of hydrogenation of benzophenone at two different concentrations of benzophenopne using IIIA/t-BuOK/2-propanol system.
146 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
7. 2). However at higher concentration of 2-propanol, the increase in conversion became
more and more insignificant, presumably due to the saturation of the catalyst with 2-propanol.
Keeping the other parameters the same and increasing the concentration of the
substrate, the rate of the reaction also increased. For instance, two experiments with 0.003
mmol of catalyst IIIA as the catalyst, 0.015 mmol of t-BuOK and 7.5 mmol of 2-propanol, but with 0.75 mmol and 1.50 mmol of benzophenone showed within 0.25 h a TOF of 619 h-1 and 912 h-1, respectively. A plot of the TOF vs time for these reactions is shown in Figure 7.3.
From the above experiments, it is evident that the reaction follows a first order kinetic
with respect to both substrate and 2-propanol. These experiments provided further evidence
for this transfer hydrogenation reaction to be operated through a primary coordination sphere
mechanism (Scheme 7.5).9a
7.4. Conclusion
In conclusion, efficient catalytic Claisen-Tishchenko reaction of formaldehyde, aliphatic
aldehydes, as well as aromatic and heteroaromatic aldehydes were realized using a nitrosyl
large bite angle Sixantphos rhenium complex along with suitable auxiliary hydrides as co-
catalyst. The active species in these transformations is expected to be the corresponding
rhenium alkoxide. In addition, mechanistically related efficient transfer hydrogenation of
aliphatic, aromatic and heteroaromatic ketones, as well as aromatic and aliphatic imines could
be achieved applying the same rhenium catalyst along with a base using 2-propanol as
hydrogen donor. A mechanism operative through an inner coordination sphere is proposed
for these transfer hydrogenation reactions.
7.5. Experimental Section
All manipulations of addition of reaction components and samplings were done in a glove box filled with dry
N2. All the reagents are purchased from either Aldrich or ABCR chemical company and used without further
purification.
147 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
General Procedure for the Claisen-Tishchenko Reaction
Complex IIIA (3 mg, 0.00296 mmol) and LiAlH4 (0.56 mg, 0.0148 mmol) was taken in a Young Schlenk tube.
THF (1 mL) was added to it and shaken for 1 min. To this, the aldehyde (appropriate quantity) was added. The
Schlenk was closed and the mixture was stirred (in the glove box at room temperature reactions or kept outside the glove box in an oil bath for heating). The reaction was monitored by GC/MS for which the samples were taken in the glove box, quenched immediately with water outside. The yield was determined by GC/MS analysis based on the consumption the aldehydes.
General Procedure for the Transfer Hydrogenation Reactions of Ketones and Imines
In a N2 glove box, complex IIIA (3 mg, 0.00296 mmol) and t-BuOK (1.67 mg, 0.0149 mmol) was taken in a
Young Schlenk tube. 2-propanol (20 equiv. with respect to the substrate) was added to it and the resulting mixture was shaken well. To this, the ketone or imine (appropriate quantity) was added. The Schlenk was closed and the mass was heated in an oil bath at 83 °C. The reaction was monitored by GC/MS for which the samples were taken into the glove box, quenched immediately with water outside the glove box. The yield was determined by GC/MS analysis based on the consumption the substrate.
GC/MS (CP-3800 Saturn 2000MS/MS spectrometer, Column: Brechbuhler, ZB-5ms, 30m x 0.25mm x 0.25µm) data (compound: retention time, (mass peak)): 1a: 3.67 min (m/z = 106); 1b: 5.05 min (m/z = 140); 1c: 6.05 min
(m/z = 136); 1d: 3.93 min (m/z = 112); 1e: 3.96 min (m/z = 112); 1f: 1.81 min (m/z = 100); 1h: 3.45 min (m/z =
112); 2a: 9.61 min (m/z = 112); 2b: 12.28 min (m/z = 280); 2c: 14.12 min (m/z = 272); 2d: 8.32 min (m/z =
224); 2e: 9.73 min (m/z = 224); 2f: 6.84 min (m/z = 200); 2h: 9.20 min (m/z = 224); 3a: 4.44 min (m/z = 120);
3e: 4.37 min (m/z = 126); 3f: 35.51 min (m/z = 182); 3h: 3.10 min (m/z = 98); 4a: 4.37 min (m/z = 122); 4e:
4.67 min (m/z = 128); 4f: 36.00 min (m/z = 184); 4h: 3.02 min (m/z = 100); 5a: 9.05 min (m/z = 181); 5b: 10.89 min (m/z = 211); 5c: 8.93 min (m/z = 199); 5d: 12.56 min (m/z = 245); 5f: 6.04 min (m/z = 163); 6a: 9.31 min
(m/z = 183); 6b: 10.97 min (m/z = 213); 6c: 9.32 min (m/z = 201); 6d: 12.96 min (m/z = 247); 6e: 10.84 min
(m/z = 217); 6f: 6.11 min (m/z = 165).
7.6. References
1. a) L. Claisen, Ber. Dtsch. Chem. Ges. 1887, 20, 646-650; b) W. Tischtschenko, Chem. Zentralbl. 1906, 77, I, 1309-1311; c) T. Seki, T. Nakajo, M. Onaka, Chem. Lett. 2006, 35, 824-829; d) M. H. S. A. Hamid, P. A. Slatford, J. M. J. Williams, Adv. Synth. Catal. 2007, 349, 1555-1575; e) K. Ekoue-Kovi, C. Wolf, Chem. Eur. J. 2008, 14, 6302-6315; f) W. I. Dzik, L. J. Gooßen, Angew. Chem. Int. Ed. 2011, 50, 11047-11049. 2. a) Ullmann’s Encyclopadia of Industrial Chemistry, 6th edn., Wiley-VCH, Weinheim, 2002.
148 Chapter 7 Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes
3. a) O.Kamm,W. F. Kamm, Org. Synth.; Coll. Vol. 1: 1941,104; b) F. W. Swamer, C. R. Hauser, J. Am. Chem. Soc. 1946, 68, 2647-2649; c) W. C. Child, H. Atkins, J. Am. Chem. Soc. 1923, 45, 3013-3023; d) Y. Ogata, A. Kawasaki, Tetrahedron 1969, 25, 929-935; e) T. Ooi, T. Miura, K. Takaya, Tetrahedron Lett. 1999, 40, 7695-7698; f) I. Simpura, V. Nevalainen, Tetrahedron 2001, 57, 9867-9872; g) T. Ooi, K. Ohmatsu, K. Sasaki, T. Miura, K. Maruoka, Tetrahedron Lett. 2003, 44, 3191-3193; h) P. R. Stupp, J. Org. Chem. 1972, 38, 1433-1434; i) Y.-S. Hon, Y.-C. Wong, C.-P. Chang, C.-H. Hsieh, Tetrahedron 2007, 63, 11325-11340; j) M. R. Crimmin, A. G. M. Barrett, M. S. Hill, P. A. Procopiou, Org. Lett. 2007, 9, 331- 333; k) B. M. Day, N. E. Mansfield, M. P. Coles, P. B. Hitchcock, Chem. Commun. 2011, 47, 4995-4997; l) M. M. Mojtahedi, E. Akbarzadeh, R. Sharifi, M. S. Abaee, Org. Lett. 2007, 9, 2791-2793; m) D. C. Waddell, J. Mack, Green Chem. 2009, 11, 79-82; n) T. Werner, J. Koch, Eur. J. Org. Chem. 2010, 6904- 6907; o) L. Cronin, F. Manoni, C. J. O’ Connor, S. J. Connon, Angew. Chem. Int. Ed. 2010, 49, 3045-3048; p) S. P Curran, S. J. Connon, Org. Lett. 2012, 14, 1074-1077. 4. K. Rajesh. H. Berke, Adv. Synth. Catal. 2013, 355, 901-906. 5. A. Chan, K. A. Scheidt, J. Am. Chem. Soc. 2006, 128, 4558-4559. 6. a) M. Yamashita, Y. Watanabe, T.-A. Mitsudo, Y. Takegami, Bull. Chem. Soc. Jpn. 1976, 49, 3597-3600; b) M. Yamashita, T. Ohishi, Appl. Organomet. Chem. 1993, 7, 357-361; c) H. Horino, T. Ito, A. Yamamoto, Chem. Lett. 1978, 7, 17-20; d) T. Ito, H. Horino, Y. Koshiro, A. Yamamoto, Bull. Chem. Soc. Jpn. 1982, 55, 504-512; e) N. Menashe, Y. Shvo, Organometallics 1991, 10, 3885-3891; f) V. V. Grushin, H. Alper, J. Org. Chem. 1991, 56, 5159-5161; g) A. Sorkau, K. Schwarzer, C. Wagner, E. Poetsch, D. Steinborn, J. Mol. Catal. A 2004, 224, 105-109; h) M.-O. Simon, S. Darses, Adv. Synth. Catal. 2010, 352, 305-308; i) M. Massoui, D. Beaupére, L. Nadjo, R. Uzan, J. Organomet. Chem. 1983, 259, 345-308; j) C. Tejel, M. A. Ciriano V. Passarelli, Chem. Eur. J. 2011, 17, 91-95; k) S. H. Bergens, D. P. Fairlie, B. Bosnich, Organometallics 1990, 9, 566-571; l) P. Barrio, M. A. Esteruelas, E. Onate, Organometallics 2004, 23, 1340-1348; m) T. Suzuki, T; Yamada, T. Matsuo, K. Watanabe, T. Katoh, Synlett 2005, 1450- 1452; n) T. Suzuki, T. Yamada, K. Watanabe, T. Katoh, Bioorg. Med. Chem. Lett. 2005, 15, 2583-2585; o) S. Ogoshi, Y. Hoshimoto, M. Ohashi, Chem. Commun. 2010, 46, 3354-3356; p) Y. Hoshimoto, M. Ohashi, S. Ogoshi, J. Am. Chem. Soc. 2011, 133, 4668-4671; q) K.-I. Morita, Y. Nishiyama, Y. Ishii, Organometallics 1993, 12, 3748-3752. 7. a) S. Onozawa, T. Sakakura, M. Tanaka, M. Shiro, Tetrahedron 1996, 52, 4291-4302; b) H. Berberich, P. W. Roesky, Angew. Chem. Int. Ed. 1998, 37, 1569-1571; c) G. B. Deacon, A. Gitlits, P. W. Roesky, M. R. Bürgstein, K. C. Lim, B. W. Skelton, A. H. White, Chem. Eur. J. 2001, 7, 127-138; d) M. R. Burgstein, H. Berberich, P. W. Roesky, Chem. Eur. J. 2001, 7, 3078-3085; e) A. Zuyls, P. W. Roesky, G. B. Deacon, K. Konstas, P. C. Junk, Eur. J. Org. Chem. 2008, 693-697; f) J.-L. Hsu, J.-M. Fang, J. Org. Chem. 2001, 66, 8573-8584; g) A. Michrowska, B. List, Nature Chem. 2009, 1, 225-228; h) T. Andrea, E. Barnea, M. S. Eisen, J. Am. Chem. Soc. 2008, 130, 2454-2455; i) M. Sharma, T. Andrea, N. J. Brookes, B. F. Yates, M. S. Eisen, J. Am. Chem. Soc. 2011, 133, 1341-1356. 8. a) T. Seki, K. Akutsu, H. Hattori, Chem. Commun. 2001, 1000-1001. 9. For reviews, see; a) S. E. Clapham, A. Hadzovic, R. H. Morris, Coord. Chem. Rev. 2004, 248, 2201–2237. b) R. Noyori, S. Hashiguchi, Acc. Chem. Res. 1997, 30, 97; c) K. Junge, K. Schröder, M. Beller, Chem.
149 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Commun. 2011, 47, 4849-4859; d) S. Chakraborty, H. Guan, Dalton Trans. 2010, 39, 7427-7436; e) J. Václavík, P. Šot, B. Vilhanová, J. Pecháček, M. Kuzma, P. Kačer, Molecules 2013, 18, 6804-6828; f) A. Bartoszewicz, N. Ahlsten, B. Martn-Matute, Chem. Eur. J. 2013, 19, 7274-7302; g) X. Wu, J. Xiao, in Hydrogenation and Transfer Hydrogenation in Water, (Metal Catalyzed Reactions in Water); 2013, 44; h) G. Brieger, T. J. Nestrick, Chem. Rev, 1974, 74, 567-580. 10. For selected examples, see; Y. Shvo, D. Czarkie, J. Organomet. Chem. 1986, 315, C25; b) Y. Blum, D. Czarkie, Y. Rahamim, Y. Shvo, Organometallics 1985, 4, 1459; c) B. L. Conley, M. K. Pennington- Boggio, E. Boz, T. J. Williams, Chem. Rev. 2010, 110, 2294; d) C. P. Casey, S. W. Singer, D. R. Powell, R. K. Hayashi, M. Kavana, J. Am. Chem. Soc. 2001, 123, 1090; e) N. Menashe, Y. Shvo, Organometallics 1991, 10, 3885; f) N. Menashe, E. Salant, Y. Shvo, J. Organomet. Chem. 1996, 514, 97; g) C. P. Casey, S. W. Singer, D. R. Powell, R. K. Hayashi, M. Kavana, J. Am. Chem. Soc. 2001, 123, 1090; h) N. Menashe, Y. Shvo, Organometallics 1991, 10, 3885; i) N. Menashe, E. Salant, Y. Shvo, J. Organomet. Chem. 1996, 514, 97. f) A. Landwehr, B. Dudle, T. Fox, O. Blacque, H. Berke, Chem Eur. J, 2012, 18, 5701-5714. 11. For selected examples, see; a) S. Hashiguchi, A. Fujii, J. Takehara, T. Ikariya, R. Noyori, J. Am.Chem. Soc. 1995, 117, 7562; b) J. X. Gao, T. Ikariya, R. Noyori, Organometallics 1996, 15, 1087; c) T. Ikariya, K. Murata, R. Noyori, Org. Biomol. Chem. 2006, 4, 393; d) R. Noyori, S. Hashiguchi, Acc. Chem. Res. 1997, 30, 97. 12. H. Berke, ChemPhysChem 2010, 11, 1837-1849. 13. a) T. Naota, H. Takaya, S.-I. Murahashi, Chem. Rev. 1998, 98, 2599; b) O. Pàmies, J.-E. Bäckvall, Chem. Eur. J. 2001, 7, 5052; c) J.-E. Bäckvall, J. Organomet. Chem. 2002, 652, 105. d) M. Yamakawa, H. Ito, R. Noyori, J. Am. Chem. Soc. 2000, 122, 1466. e) S. Gladiali, G. Mestroni, Transit. Met. Org. Synth. 1998, 2, 97; f) J. S. M. Samec, J.-E. Bäckvall, Chem. Eur. J. 2002, 8, 2955-2961. 14; g) A. Aranyos, G. Csjernyik, K. J. Szabó, J.-E. Bäckvall, Chem. Commun. 1999, 351-352; ibid, 2131; h) E. Mizushima, M. Yamaguchi, T. Yamagishi, J. Mol. Catal. A: Chem. 1999, 148, 69; i) C. Vicente, G. B. Shulpin, B. Moreno, S. Sabo- Etienne, B. Chaudret, J. Mol. Catal. A: Chem. 1995, 98, L5-L8; j) P. A. Chaloner, M.A. Esteruelas, F. Joó, L. A. Oro, in Homogeneous Hydrogenation (Ch. 3), Kluwer Academic Publishers, Dordrecht, The Netherlands,1994; k) E. Mizushima, M. Yamaguchi, T. Yamagishi, Chem. Lett. 1997, 237-238; l) C. Standfest-Hauser, C. Slugovc, K. Mereiter, R. Schmid, K. Kirchner, L. Xiao, W. Weissensteiner J. Chem. Soc. Dalton Trans. 2001, 2989; m) Y. Nishibayashi, I. Takei, S. Uemura, M. Hidai, Organometallics 1999, 18, 2291; n) P. Dani, T. Karlen, R.A. Gossage, S. Gladiali, G. van Koten, Angew. Chem. Int. Ed. Engl. 2000, 39, 743. o) G. C. Jia, H. M. Lee, L. D. Williams, J. Organomet. Chem. 1997, 534, 173; p) H. Yang, M. Alvarez, N. Lugan, R. Mathieu, J. Chem. Soc. Chem. Commun. 1995, 1721; s) M. S. Rahman, P.D. Prince, J.W. Steed, K.K. Hii, Organometallics 2002, 21, 4927; q) A.C. Benyei, F. Joó, J. Mol. Catal. 1990, 58, 151; r) A. Caballero, F. A. Jalon, B.R. Manzano, J. Chem. Soc., Chem. Commun. 1998, 1879; s) D. Sellmann, F. Geipel, M. Moll, Angew. Chem. Int. Ed. Engl. 2000, 39, 561; t) S. Bhaduri, K. Sharma, D. Mukesh, J. Chem. Soc. Dalton Trans. 1993, 1191; u) E.M. Gordon, D.C. Gaba, K. A. Jebber, D.M. Zacharias, Organometallics 1993, 5020; v) C.S. Yi, Z. He, I. A. Guzei, Organometallics 2001, 20, 3641. .
150 Chapter 8 Homogeneous Thermocontrolled Chemoselective Tranfer Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
Homogeneous Thermocontrolled Chemoselective Transfer Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
8.1. Introduction
Amines are an important class of compounds constituting organic building blocks and
intermediates, in pharmaceuticals, textile, rubber and other agrochemicals as well as in
biological processes leading them to be significant in academia and industry.1 The production
of amines are often achieved by catalytic reduction of nitro compounds and imines, amination
of alcohols over acidic catalysts, Hydroamination of alkenes, reductive amination of carbonyl
compounds, treatment of primary amines with alkyl halides or dialkyl sulfates or sulfonates,
addition of nucleophiles or radicals to N-substituted imines.2 Understanding the mechanism and thereby attempting tuning of the ligand-sphere, catalytic homogeneous hydrogenation and transfer hydrogenation processes are promising tools for the production of a variety of amine compounds. Nitriles are frequently available substrates and the catalytic hydrogenation or transfer hydrogenation of them to produce amines is a challenging difficult process,3 which is reflected in the very limited number of reports, particularly for the later process for which active systems could be developed only very recently.4,5 Like catalytic hydrogenation, catalytic transfer hydrogenation is also considered to be an environmentally friendly transformation, the latter even much safer and convenient to handle on any scale. The transfer hydrogenation of nitriles followed by subsequent N-monoalkylation to secondary amines was also reported recently.6 Like the catalytic hydrogenation of nitriles, the transfer hydrogenation reactions also suffer from selectivity problems (Scheme 8.1).6
151 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
8.2. Results and Discussion
Transfer Hydrogenation of Nitriles Catalyzed by IIIA
In Chapter 3, we have already discussed the activity of complex IIIA for the hydrogenation of nitriles showing selectivity towards secondary or tertiary amines depending on the structure of the nitrile. Also, the ability of this complex to induce transfer hydrogenation reactions of ketones and imines in the presence of suitable bases were also discussed (Chapter
7). We then tested the activity of this complex in transfer hydrogenations of nitriles (Table
8.1). With loadings of 1 mol% of IIIA and 3 mol% t-BuOK in 25 equiv. of 2-propanol at 60
°C, the transfer hydrogenation of benzonitrile was effected giving a yield of 6% of
benzylamine (2a) and 58% of N-isopropylidenebenzylamine (3a) and < 1% of N-
benzylisopropylamine (4a) within 12 h of reaction time (Table 8.1, entry 1). N-
benzylideneisopropylamine (3a) was formed by the condensation reaction between the
formed benzylamine (2a) and acetone (Scheme 8.1). N-benzylisopropylamine (4a) has to be
considered a follow-up product of transfer hydrogenation of imine 2a. This reaction, when
carried out at 83 °C, showed the formation of 49% of 2a, 47% of 3a (overall 96%; TOF: 384
h-1) and < 1% of 4a in 0.25 h (Table 8.1, entry 2). 3a can be considered as the primary amine
component since 3a can be hydrolysed to 2a in the work up procedure. Thus, the reaction
Scheme 8.1. Selective Transfer hydrogenation as well as reductive alkylation of nitriles to amines and N- alkylisopropylamine respectively using IIIA/t-BuOK/2-propanol system.
152 Chapter 8 Homogeneous Thermocontrolled Chemoselective Tranfer Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
(Table 8.1, entry 2), upon an acid-base work up following purification by column
chromatography, furnished 91% yield of benzylamine (2a) (Table 8.1, entry 2, in
parentheses). Another run of this reaction under the same conditions sampled at 1 h gave 9%
of 2a, 86% of 3a and 4% of 4a (Table 8.1, entry 3). Under the same loading of the reaction components, but carried out at a the higher temperature of 140 °C, the reaction furnished the reductively alkylated product 4a in 93% yield when run for 12 h (Table 8.1, entry 4). Thus, a
temperature control can be applied to furnish the desired products of either 2a or 4a. In order
to exclude the alkylated products, the reaction with 2-butanol was carried out at 100 °C,
which did not give any of the desired products, instead 5% of N-benzylidenebenzylamine
(PhCH=NCH2Ph) was observed after 1 h.
This transfer hydrogenation strategy using 2-propanol as H2 donor has been extended
to a few more aromatic and aliphatic nitriles (Table 8.1). 3-methylbenzonitrile (1b) could be
smoothly converted to the corresponding primary amine 2b and the imine 3b in yields of
41% and 58% respectively, (overall 99%; TOF: 396 h-1) in 0.25 h when a loading of 1 mol%
of IIIA was adopted at 83 °C (Table 8.1, entry 5). 4-methylbenzonitrile (1c) under these
conditions furnished a yield of 36% of primary amine 2c and 58% of the imine 3c (overall
94%; TOF: 376 h-1) in 0.25 h (Table 8.1, entry 6). As expected, this reaction after one hour showed a decrease in primary amine 2c to 9% and an increase in imine 3c to 87% (overall
96%) (Table 8.1, entry 7). 4-methoxybenzonitrile (1d) showed no primary amine 2d, but furnished 63% of the imine 3d (TOF: 252 h-1) in 0.25 h when a catalyst loading of 1 mol% was adopted (Table 8.1, entry 8). The absence of this electron rich primary amine 2d is not surprising, since it is expected that this amine would immediately react with acetone to form the imine 3d. This reaction gave a yield of 83% of imine 3d when run for 1 h (Table 8.1, entry 9). When this reaction mixture was heated to 140 °C for 16 h, only 69% of the reductively alkylated product 4d was obtained, the remains being the imine 3d (Table 8.1,
153 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
entry 10). 2-bromobenzonitrile (1e) under the conditions of 1 mol% loading of IIIA at 83 °C, furnished 64 % of primary amine 2e and 34% of imine 3e (overall 98%) when run for 0.25 h
(Table 8.1, entry 11). Under the same conditions, 4-bromobenzonitrile (1f) furnished 69 % of primary amine 2f and 28% of imine 3e (overall 97%) when run for 0.25 h (Table 8.1, entry
12). However, the reaction mixtures of both 3-(trifluoromethyl)benzonitrile (1g) and 3-
(phenoxy)benzonitrile (1h), after 99% conversion to a mixture of 2g-h and 3g-h in 0.25 h
(TOF (2g+3g) and (2h+3h), both 396 h-1) (Table 8.1, entries 13 and 15) when heated to 140
°C for 16 h furnished the corresponding reductively alkylated products 4g and 4h, respectively, in quantitative yield (Table 8.1, entries 14 and 16). A higher loading of 2 mol% of IIIA was adopted for the transfer hydrogenation of the electron deficient aromatic nitrile,
3,4-difluorobenzonitrile(1i) which gave 72% of 2i and 24% of 3i (overall 96%; TOF: 192 h-1)
in 0.25 h (Table 8.1, entry 17). Similarly, a loading of 2 mol% of IIIA was adopted for the
heteroaromatic nitrile, thiophene-2-carbonitrile (1j), which furnished 7% of the
corresponding primary amine (2j) , 83% of the imine (3j) (overall 99%; TOF: 83 h-1 (first h))
when run for 3 h (Table 8.1, entries 18 and 19). Catalyst loading of 2.5 mol% was adopted for
the transfer hydrogenations of the aliphatic nitrile, cyclohexanecarbonirile (1k) which
showed 5% yield of the primary amine 2k and 70% yield of the imine 3k (overall 75%; TOF:
Table 8.1. Transfer hydrogenation of various nitriles catalyzed by IIIA/t-BuOK/2-propanol system.
Entry Nitrile Cat TOF/ Temp/ °C Time Yield (%)b Conv. 1 (mol%) (h-1)a (h) 2/3/4 (%)
1 1 - 60 12 6/58/< 1 65 2 1 384 83 0.25 49/47/< 1 (91) 97 3 1a - 83 1 9/86/4 99 4 1 - 140 16 0/1/93 99
154 Chapter 8 Homogeneous Thermocontrolled Chemoselective Tranfer Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
5 1b 1 396 83 0.25 41/58/< 1 > 99
6 1c 1 376 83 0.25 36/58/0 94 7 1 9/87/1 97
8 1 252 83 0.25 0/63/0 63 9 1d - 1 0/83/2 85 10 140 16 0/31/69 100
11 1e 1 392 83 0.25 64/34/0 98
12 1f 1 388 83 0.25 69/28/2 > 99
13 1g 1 396 83 0.25 63/36/0 99 14 140 16 0/0/> 99 > 99
15 1h 1 396 83 0.25 45/54/0 99 16 - 140 16 0/0/> 99 > 99
17 1i 2 192 83 0.25 72/24/0 96
18 1j 2 98 83 0.25 32/17/0 49 19 - 3 7/83/0 90
20 1k 2.5 120 83 0.25 5/70/0 75 21 - 1 9/80/5 94
22 1l 2 182 83 0.25 18/67/2 91d 23 - 1 5/83/3 95d aTOF for the formation of 2 and 3. bUnless mentioned, yield by GC/MS based on the consumption of nitrile. cIsolated yield of benzylamine. d Remaining being unidentified products.
155 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
120 h-1) in 0.25 h (Table 8.1, entry 20). When run for 1 h, the reaction could give rise to 2k
and 3k in yield of 9% and 80% respectively (overall 89%) (Table 8.1, entry 21). The relatively higher quantity of imine 3k is attributed to be due to the electron richness of this
nitrile 1k, which would enhance the formation of the imine 3k. As an effect of this higher
quantity of the imine 3k, a relatively higher (5%) yield of the reductive alkylated product was
also observed (Table 8.1, entry 21). Under this conditions, but with a loading of 2 mol% of
IIIA, phenylacetonitrile (1l) showed a yield of 18% of the amine 2l and 67% of the imine 3l
(overall 85%; TOF: 182 h-1) in 0.25 h (Table 8.1, entry 22). This reaction run for 1 h could
give rise to 5% yield of amine 2l and 83% yield of the imine 3l (overall 88%) (Table 8.1,
entry 23).
8.3. Mechanistic Aspects
The mechanism of this transfer hydrogenation of nitriles is assumed to be operative through a
Scheme 8.2. Proposed mechanism for the transfer hyfrogenation of nitriles.
156 Chapter 8 Homogeneous Thermocontrolled Chemoselective Tranfer Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
catalytic cycle analogous to the one described for transfer hydrogenations of ketones in
Chapter 7. The active species is proposed to be the rhenium dihydrides species XXVIIA
(Scheme 8.2). The aldimine (RCH=NH) generated would further undergo transfer
hydrogenation reaction by entering into step 1 followed by an analogous catalytic cycle,
would yield the primary amine (RCH2NH2).
8.4. Conclusion
The transfer hydrogenation reactions of a series of aromatic, heteroaromatic and aliphatic
nitriles have been realized using IIIA/t-BuOK/2-propanol system in high yields. Carrying out
these reactions at a temperature of 83 °C furnished a mixture of primary amines 2 and N-
isopropylidenebenzylamines 3, the latter can easily be hydrolyzed to the primary amines 2.
Carrying out the transfer hydrogenation reactions at elevated temperature of 140 °C at the
beginning itself or after the complete formation of primary amines and N-isopropylaldimines
furnished the reductive alkylation products, N-isopropylalkylamines 4 in good to excellent
yields. The mechanism of these transfer hydrogenation reactions of nitriles is also proposed.
8.5. Experimetal Section
All manipulations of addition of reaction components and samplings were done in a glove box filled with dry
N2. All the reagents are purchased from either Aldrich or ABCR chemical company and used without further purification.
General Procedure for the Transfer Hydrogenation of Nitriles
Catalyst IIIA (0.003 mg, 0.00296 mmol) and t-BuOK (0.001 mg, 0.0089 mmol) were taken in a 5 mL Young schlenk flask. To this, benzonitrile (30.5 µL, 0.296 mmol) and 2-propanol (0.57 mL, 0.7.41 mmol) were added.
The flask was closed and kept in an oil bath maintained at 83 °C. After appropriate reaction time, the mass was diluted with dichloromethane and analyzed by GC/MS. The yields of the products were determined based on the consumption of nitrile.
157 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
For reactions to furnish the reductive alkylated products 4, either the flask containing the reactants was kept in an oil bath maintained at 140 °C or the reaction mass (after the appropriate time of reaction at 83 °C and sampling) was kept in an oil bath maintained at 140 °C. After appropriate reaction time, the samples were analyzed by GC/MS (CP-3800 Saturn 2000MS/MS spectrometer, Column: Brechbuhler, ZB-5ms, 30m x
0.25mm x 0.25µm) and the yield of the product was determined based on the consumption of the nitrile.
GC/MS data (compound: retention time (mass peak)): 1a: 3.78 min (m/z = 103); 2a: 4.00 min (m/z = 107); 3a:
5.85 min (m/z = 147); 4a: 5.26 min (m/z = 149); 1b: 4.66 min (m/z = 117); 2b: 4.86 min (m/z = 121); 3b: 6.61 min (m/z = 161); 4b: 6.02 min (m/z = 163); 1c: 4.78 min (m/z = 117); 2c: 4.88 min (m/z = 121); 4b: 6.68 min
(m/z = 161); 4c: 6.02 min (m/z = 163); 1d: 6.12 min (m/z = 137); 3d: 7.77 (m/z = 177); 4d: 7.22 min (m/z =
179); 1e: 6.01 min (m/z = 181); 2e: 6.24 min (m/z = 185); 3e: 7.72 min (m/z = 225); 1f: 5.89.02 min (m/z =
181); 2f: 6.47 min (m/z = 185); 3f: 8.71 min (m/z = 225); 4f: 7.56 min (m/z = 227); 1g: 3.53 min (m/z = 171);
2g: 4.23 min (m/z = 175); 3g: 5.81 min (m/z = 215); 4g: 5.28 min (m/z = 217); 1h: 9.24 min (m/z = 195); 2h:
9.62 min (m/z = 199); 3h: 10.80 min (m/z = 239); 4h: 10.34 min (m/z = 241); 1i: 3.47 min (m/z = 139); 2i: 4.29
(m/z = 143); 3i: 5.98 min (m/z = 183); 1j: 3.86 min (m/z = 109); 2j: 4.03 min (m/z = 113); 3j: min (m/z = 153);
1k: 3.75 min (m/z = 109); 2k: 3.77 min (m/z = 113); 3k: 5.32 min (m/z = 153); 4k: 4.98 min (m/z = 155); 1l:
5.03 min (m/z = 117); 2l: 4.72 min (m/z = 121); 3l: 6.21 min (m/z = 161); 4l: 5.54 min (m/z = 163);
8.6. References
1. For selected reviews and highlights, a) T. E. Müller, M. Beller, Chem. Rev. 1998, 98, 675-703; b) M. J. Palmer, M. Wills, Tetrahedron: Asymmetry 1999, 10, 2045-2061; c) J. Seayad, A. Tillack, C. G. Hartung, M. Beller, Adv. Synth. Catal. 2002, 344, 795-813; d) F. Pohlki, S. Doye, Chem. Soc. Rev. 2003, 32, 104-114; e) W. Tang, X. Zhang, Chem. Rev. 2003, 103, 3029; f) H.-U. Blaser, F. Spindler in Handbook of Homogeneous Hydrogenation, Vol. 3 (Eds.: J. G. de Vries, C. J. Elsevier), Wiley-VCH, Weinheim, 2007, pp. 1193; g) R. Severin, S. Doye, Chem. Soc. Rev. 2007, 36, 1407-1420; h) T. E. M_ller, K. C. Hultzsch, M. Yus, F. Foubelo, M. Tada, Chem. Soc. Rev. 2008, 37, 3795-3892; i) T. C. Nugent, M. EI-Shazly, Adv. Synth. Catal. 2010, 352, 753-819; j) M. Rueping, E. Sugiono, F. R. Schoepke, Synlett 2010, 852 -865. 2. a) Handbook of Homogeneous Hydrogenation, (Eds.: J. G. de Vries, C. J. Elsevier), Wiley-VCH, Weinheim, 2007, b) K. S. Hayes, Appl. Catal. A: Gen. 2001, 221, 187-195. c)A. Seayad, M. Ahmed, H. Klein, R. Jackstell, T. Gross, M. Beller, Science 2002, 297, 1676-1678. 3. a) R. A. Grey, G. P. Pez, A. Wallo, J. Am. Chem. Soc. 1981, 103, 7536-7542; b) T. Yoshida, T. Okano, S. Otsuka, J. Chem. Soc., Chem. Commun. 1979, 870-871. 4. a) T. Yoshida, T. Okano, S. Otsuka, J. Chem. Soc. Chem. Commun. 1979, 870-871; b) R. A. Grey, G. P. Pez, A. Wallo, J. Am. Chem. Soc. 1981, 103, 7536-7545; c) T. Suarez, B. Fontal, J. Mol. Catal.
158 Chapter 8 Homogeneous Thermocontrolled Chemoselective Tranfer Hydrogenations of Nitriles Catalyzed by Rhenium Complexes
1988, 45, 335-344; d) C. S. Chin, B. Lee, Catal. Lett. 1992, 14, 135-140; e) A. M. Joshi, K. S. MacFarlane, B. R. James, P. Frediani in Progress in Catalysis (Eds.: K. J. Smith, E. C. Sanford), Elsevier, New York, 1992, pp. 143-146; f) A. M. Joshi, K. S. MacFarlane, B. R. James, P. Frediani, Chemical Industries, Vol. 53, Catalysis of Organic Reactions, Dekker, New York, 1992, 143-146; g) B. Fontal, M. Reyes, T. Surez, F. Bellandi, N. Ruiz, J. Mol. Catal. A: Chem. 1999, 149, 87-97; h) C. Bianchini, V. Dal Santo, A. Meli, W. Oberhauser, R. Psaro, F. Vizza, Organometallics 2000, 19, 2433- 2444; i) X. Xie, C. L. Liotta, C. A. Eckert, Ind. Eng. Chem. Res. 2004, 43, 7907-7911; j) T. Li, I. Bergner, F. N. Haque, M. Zimmer-De Iuliis, D. Song, R. Morris, Organometallics 2007, 26, 5940- 5949; k) S. Enthaler, K. Junge, D. Addis, G. Erre, M. Beller, ChemSusChem 2008, 1, 1006-1010; l) S. Enthaler, D. Addis, K. Junge, G. Erre, M. Beller, Chem. Eur. J. 2008, 14, 9491-9494; m) D. Addis, S. Enthaler, K. Junge, B. Wendt, M. Beller, Tetrahedron Lett. 2009, 50, 3654-3656; n) R. Reguillo, M. Grellier, N. Vautravers, L. Vendier, S. Sabo-Etienne, J. Am. Chem. Soc. 2010, 132, 7854-7855; o) C. Gunanathan, M. Hçlscher, W. Leitner, Eur. J. Inorg. Chem. 2011, 3381 –3386; p) Dipankar Srimani, Moran Feller, Yehoshoa Ben-David and David Milstein; Chem. Commun. 2012, 48, 11853-11855. 5. S. Werkmeister, C. Bornschein, K. Junge, M. Beller, Chem. Eur. J. 2013, 19, 4437-4440; b) E. Mizushima, M. Yamaguchi, T. Yamagishi, J. Mol. Catal. A: Chem. 1999, 148, 69–75. 6. X. Cui, Y. Zhang, F. Shi, Y. Deng, Chem. Eur. J. 2011, 17, 2587-2591; b) S. Werkmeister, C. Bornschein, K. Junge, M. Beller, Eur. J. Org. Chem. 2013, 3671-3674; c
159 Chapter 9 Homogeneous Hydrosilylations of Nitriles Catalyzed by Rhenium Complexes
Homogeneous Hydrosilylations of Nitriles Catalyzed by Rhenium Complexes
9.1. Introduction
The Nitrile group is one of the most difficult functionality of organic compounds to undergo
1 reduction. The hydrosilylation of nitriles to furnish N-silylaldimines RCH=NSiR’3 is a synthetically useful method, but there are only very few catalytic systems documented in literature, which reflect this challenging aspect of 1,2-addition to C≡N bonds.2 Like the hydrogenation and transfer hydrogenation of nitriles, the hydrosilylation reaction also involves a chemoselectivity issue since the N-silylaldimines, are often much more reactive than nitriles and thus the former would undergo further reactions to give N,N-disilylamines
2d,3 4 RCH2N(SiR’3)2. There are a few reports on stoichiometric hydrosilylation of nitriles.
Most of the catalytic reactions reported on the hydrosilylation of nitriles suffer from harsh reaction conditions, very slow rates, low yields and there in almost no reaction for the hydrosilylation of aliphatic nitriles.5 Nikonov and co-workers have recently documented a promising Ru system operative for chemoselective hydrosilylation of nitriles.6 However, in any of the reports, the direct synthetic accessibility of the N-silylaldimines could not be established.
9.2. Results and Discussion
9.2.1. Hydrosilylation of Nitriles Catalyzed by IIIA
Having encouraged by the ability of the complex IIIA to impart catalytic hydrogenations and transfer hydrogenataion of a variety of substrates including nitriles, we
160 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
tested the activity of complex IIIA for the hydrosilylation of nitriles. With 1 mol% of loading
of IIIA at 80 °C in THF, the hydrosilylation of benzonitrile was effected using 1.05 equiv. of
Et3SiH giving rise to 79% yield of the monosilylated product N-(triethylsilyl)benzaldimine
(2a) when run for 1 h (Table 9.1, entry 1) (Scheme 9.1). Under these conditions and run for a
period of 1 h, a screening of solvents was carried out. Pentane did not support any reaction; in
dichloromethane only 2% of the desired product 2a was formed. Reactions in the ether, t-
BuOMe furnished 67% of the product 2a and that in chlorobenzene led to a yield of 78%.
Scheme 9.1. Hydrosilylation of nitriles and synthetic utility of the formed silylimines for the production of higher imines/amine protection.
a Table 9.1. Solvent screening for the hydrosilylation of benzonitrile using Et3SiH.
IIIA (1 mol%) SiEt3 PhCN + Et3SiH Ph N 80 oC 1a 2a
Entry Solvent Yieldb (2, %) 1 THF 79 2 Pentane 0 3 Dichloromethane 2 4 t-BuOMe 67 5 Chlorobenzene 78 6 Toluene 93 a 1 mol% of complex IIIA and 105 mol% of Et3SiH were used. bBy GC/MS based on the consumption of benzonitrile.
However, toluene was found to the best solvent among the tested ones, which furnished 93% of the desired product 2a. Continuing this reaction gave 98% yield of 2a in less than 1.25 h
161 Chapter 9 Homogeneous Hydrosilylations of Nitriles Catalyzed by Rhenium Complexes
(Table 9.2, entry 1). When carried out with a higher amount of 2.1 equiv. of Et3SiH, this reaction in toluene gave selectively the monosilylated product 2a in 99% yield (Table 9.2,
entry 3). However, no reaction was observed with PhSiH3, Ph2SiH2, Ph3SiH, PhMeSiH2,
Ph2MeSiH and (MeO)3SiH. At even the higher temperature of 120 °C, the reaction was attempted with Ph2SiH2 and (MeO)3SiH, but no reaction was found to proceed.
To underline the generality of the hydrosilylation reaction using IIIA as a catalyst, a variety of nitriles were tested with Et3SiH (1.05 equiv.) in toluene, most of which could give excellent yields when carried out at a temperature of °C. N-(triethylsilyl)benzaldimine (2a), obtained by the hydrosilylation of benonitrile with a yield of 78% (TOF of 98 h-1) (Table 9.2, entry 1), upon addition of 1 equiv. of benzylamine 3 (R = PhCH2) (Scheme 9.1) at room temperature immediately furnished the imine N-benzylidinebenzylamine 4 (Ar = Ph, R =
PhCH2) in quantitative yield with respect to 2a or 98% yield with respect to 1a (Table 9.2, entry 2). The other product of this type of reaction was found to be triethylsilylamine
(Et3SiNH2) (5). 5 was found to react with primary amines to form N-(triethylsilyl)amines (7) in as reflected in the addition of more than one equiv. of benzylamine in the hydrosilylation reaction of 3-toluonitrile (1b) (Table 9.2, entries 5 and 6), furnishing 7 in 56% yield (with respect to 2b) (Table 9.2, entry 6). Under the optimized conditions, both 3-tolunitrile (1b) and
4-tolunitrile (1c) furnished 98% yield of the desired products 2b and 2c with a TOF of 78 h-1 and 65 h-1 respectively (Table 9.2, entries 4 and 7). 2-bromobenzonitrile (1d) showed under these conditions a TOF of 65 h-1 giving 98% yield of the desired product 2d (Table 9.2, entry
8) where as 4-bromobenzonitrile (1e) required a comparatively higher loading of 2 mol% of
IIIA furnishing 97% yield of the desired product 2e within 1.5 h (Table 9.2, entry 10). The electron deficient 3-trifluoromethylbenzonitrile (1f) could give a yield of only 42% of 2f in 5 h when a loading of 2 mol% of IIIA was adopted (Table 9.2, entry 11). However, under the
162 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
optimized conditions, both 3,4-difluorobenzonitrile (1g) and 3-chloro-4-fluorobenzonitrile
(1h) could be smoothly converted to their corresponding N-silylaldimines 2g and 2h in yields
of 98% and 95% in 1 h and 1.25 h, respectively (TOF: 98 h-1 (2g); 66 h-1 (2h)) (Table 9.2,
entries 12 and 15). The strategy of amine additions were tested also for these reactions. The
reaction mixture containing 2g upon addition of one equiv. of benzylamine 3 gave rise to the imine 4g (Ar = 3,4-difluorophenyl, R = PhCH2) in 43% yield (with respect to 2g) at room
temperature (Table 9.2, entry 13). This upon addition of one more equiv. of benzylamine
furnished 98% of the imine 4g (with respect to 1g) along with 79% of N-
(triethylsilyl)benzylamine (7) (with respect to 2g) (Table 9.2, entry 14). When 2 equiv. of
aniline was added to the reaction mixture containing 2h, a 95% yield of the imine 4h and
Table 9.2. Hydrosilylation of various nitriles with Et3SiH and subsequent reaction of the formed N- silylaldiminesa
Entry Nitrile IIIA TOF/ Time Yield Conv. 1 (mol%) (h-1)[a] (h) (%) (%)
1 1 78 1.25 98 98
2 1a + PhCH2NH2 98 (4a) - 99b 3 1 79 1.25 99
4 1 65 1.5 98 98
5 1b + PhCH2NH2 98 (4b) -
6 + PhCH2NH2 56 (7) -
7 1c 1 78 1.25 98 98
8 1d 1 65 1.5 98 98
1 12 5 58 58 9 1e 2 32 1.5 97 97 10
163 Chapter 9 Homogeneous Hydrosilylations of Nitriles Catalyzed by Rhenium Complexes
11 1f 2 4 5 42 68c
12 1 98 1 98 98
13 1g + PhCH2NH2 43 (4g) -
14 + PhCH2NH2 98 (4g), 79 (7) -
1 66 1.25 95 99c 15 1h + 2 PhNH2 95 (4h), 79 (7) - 16
17 1i 2 2 2 88 98c
18 1j 2 8 2 33 41c,d
aAll reactions were carried out in toluene, yield by GC/MS based on the consumption of benzonitrile. b c d Reaction with 2.1 equiv. of Et3SiH. Remaining being unidentified product(s). Reaction was carriedout at 120 °C.
79% yield of 7 were obtained (Table 9.2, entry 16). A loading of 2 mol% of IIIA was adopted for the hydrosilylation of the heteroaromatic nitrile, 2-thiophenecarbonitrile (1i) showed a TOF of 22 h-1 giving rise to 88% yield of the desired N-silylaldimine 2i within 2 h
(Table 9.2, entry 17). However, under these conditions, the aliphatic nitrile phenylacetonitrile
(1j) could give only 33% yield of the desired N-silylialdmine 2j in 2 h (Table 9.2, entry 18).
9.3. Mechanistic Aspects
It was already discussed in Chapter 2 that the species VA can be obtained via the reaction of
IIIA with Et3SiH. It is a well known fact that the role of R3SiH in hydrosilylations is related to the role of H2 in hydrogenations. Therefore, we anticipated a mechanism of the hydrosilylation of nitriles similar to the one proposed for the hydrogenation of olefins.
164 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Primary coordination of nitrile to the coordinatively unsaturated species V (Scheme 9.2) would generate the 18e species XXXVA. In a cis addition mode β-hydride transfer occurs to give the coordinatively unsaturated species XXXVIA. Coordination of Et3SiH, followed by oxidative addition of it, would generate the Re(III) species XXXVIIA. This would then upon reductive elimination of the product 2 would regenerate the active species VA.
P
O = P
Scheme 9.2. Proposed mechanism of hydrosilylation of nitriles catalyzed by IIIA.
9.4. Conclusion
Efficient catalytic hydrosilylation of aromatic nitriles using Et3SiH could be realized applying catalyst IIIA. TOFs of up to 98 h-1 were achieved at 80 °C with almost equimolar ratios of nitriles and Et3SiH as starting materials. Only mono hydrosilylation was observed even when
2.1 equiv. of Et3SiH was present. The hydrosilylation reaction of aliphatic nitriles was not as efficient as that of aromatic nitriles. For the first time, the synthetic utility of a hydrosilylation
165 Chapter 9 Homogeneous Hydrosilylations of Nitriles Catalyzed by Rhenium Complexes
reaction could be demonstrated. This produced N-benzylidineamines, R3SiNH2 and N- silylamines in a one-pot fashion. Though a variety of compounds including amino acids can be synthesized by the application of nucleophilic addition to N-silylaldimines, the literature in this area is scarce mainly due to the difficult synthetic accessibility of N-silylaldimines.7
9.5. Experimental Section
All operations were done in a glove box filled with dry N2 gas. Only the properly clsed reaction vessels were kept outside in an oil bath at the temperature mentioned.
General Procedure for the Hydrosilylation of Nitriles
Catalyst IIIA (0.003 mg, 0.00198 mmol) was weighed into a 5 mL Young schlenk flask. Benzonitrile (20.4 µL,
0.198 mmol) and Et3SiH (20.7 µL, 0.31 mmol) were added to it using a micro pipette. Toluene (0.5 mL) was added to it. The flask was closed and kept in an oil bath maintained at 80 °C. After appropriate reaction time, the mass was diluted with dry dichloromethane and analyzed by GC/MS. The yields of the products were determined based on the consumption of nitrile.
Procedure for the Subsequent Addition of Amines
The amines were directly added at room temperature to the reaction mass obtained after hydrosilylation reactions. All the products were analyzed by GC/MS and yields were calculated based on the hydrosilylated product.
GC/MS Data: 1a: 3.79 min (m/z = 103); Et3SiH: 2.99 min (m/z = 116); Benzylamine: 4.02 min (m/z = 107);
Aniline: 3.70 min (m/z: 93); Et3SiNH2 (5): 2.80 min (m/z (-EtH): 102); 2a: 8.06 min (m/z = 219); 4a: 9.46 min
(m/z = 195); 7 (R = Ph): 7.77 min (m/z = 207); 7 (R = PhCH2): 8.08 min (m/z = 221); 1b: 4.64 min (m/z =
117); 2b: 8.67 min (m/z = 233); 1c: 4.76 min (m/z = 117); 2c: min (m/z = 233); 1d: 6.24 min (m/z = 181); 2d:
9.56 min (m/z = 299); 1e: 5.89 min (m/z = 181); 2e: 9.91 min (m/z = 299); 1f: 3.52 min (m/z = 171); 2f: 3.79 min (m/z = 287); 1g: 3.47 min (m/z = 137); 2g: 7.93 min (m/z = 255); 1h: 4.91 min (m/z = 155); 1a: 9.18 min
(m/z = 271); 1i: 3.86 min (m/z = 109); 1i: 8.20 min (m/z = 225); 1j: 5.01 min (m/z = 117); 1i: 9.79 min (m/z =
181).
9.6. References
1. a) R. A. Grey, G. P. Pez, A. Wallo, J. Am. Chem. Soc. 1981, 103, 7536-7542; b) T. Yoshida, T. Okano, S. Otsuka, J. Chem. Soc., Chem. Commun. 1979, 870-871
166 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
2. a) I. Ojima in The Chemistry of Organic Silicon Compounds (Eds.: S. Patai, Z. Rappoport), Wiley, New York, 1989, ch. 25. b) R. Calas, Pure Appl. Chem. 1966, 13, 61; c) A. J. Chalk, J. Organomet. Chem. 1970, 21, 207; d) R. J. P. Corriu, J. J. E. Moreau, M. Pataud-Sat, J. Organomet. Chem. 1982, 228, 301; e) B. Marciniec, Comprehensive Handbook on Hydrosilylation, Pergamon, Oxford, 1992. 3. a) T. Murai, T. Sakane, S. Kato, J. Org. Chem. 1990, 55, 449-453; b) T. Murai, T. Sakane, S. Kato, Tetrahedron Lett. 1985, 26, 5145-5148; c) A. M. Caporusso, N. Panziera, P. Petrici, E. Pitzalis, P. Salvadori, G. Vitulli, G. Martra, J. Mol. Catal. A 1999, 150, 275-285. 4. a) J. Kim, Y. Kang, J. Lee, Y. K. Kong, M. S. Gong, S. O. Kang, J. Ko, Organometallics 2001, 20, 937; b) M. Tanabe, K. Osakada, Organometallics 2001, 20, 2118; c) H. Hashimoto, I. Aratani, C. Kabuto, M. Kira, Organometallics 2003, 22, 2199; d) T. Watanabe, H. Hashimoto, H. Tobita, J. Am. Chem. Soc. 2007, 128, 2176; e) M. Ochiai, H. Hashimoto, H. Tobita, Angew. Chem. Int. Ed. 2007, 46, 8192. 5 a) A. Y. Khalimon, R. Simionescu, L. G. Kuzmina, J. A. K. Howard, G. I. Nikonov, Angew. Chem. Int. Ed. 2008, 47, 7701; b) E. Peterson, A. Y. Khalimon, R. Simionescu, L. G. Kuzmina, J. A. K. Howard, G. I. Nikonov, J. Am. Chem. Soc. 2009, 131, 908. 6. D. V. Gutsulyak, G. I. Nikonov, Angew. Chem. 2010, 122, 7715-7718. 7. a) G. Cainelli, M. Panunzio, P. Andreoli, G. Martelli, G. Spunta, D. Giacomini, E. Bandini, Pure Appl. Chem. 1990, 62, 605-612; b) S. Itsuno, M. Sasaki, S. Kuroda, K. Ito, Tetrahedron: Asymmetry. 1995, 6, 1507-1510); G. Cainelli, D. Giacomini, E. Mezzina, M. Panunzio, P. Zarantonello, Tetrahedron Lett. 1991, 32, 2967-2970.
167 COMMUNICATIONS
DOI: 10.1002/adsc.201200970 Alkali Metal tert-Butoxides, Hydrides and Bis(trimethylsilyl)amides as Efficient Homogeneous Catalysts for Claisen–Tishchenko Reaction
Kunjanpillai Rajesha and Heinz Berkea,* a Institute of Inorganic Chemistry, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland Fax(ACHTUNGTRENNUNG +41)-1-635-6802 E-mail: [email protected]
Received: November 3, 2012; Published online: March 15, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200970.
Abstract: Shelf-available alkali metal tert-butoxides, attracted wide attention for the last century and is ap- hydrides and bis(trimethylsilyl)amides were shown plied in the food, polymer, dye and perfume industry to be highly efficient homogeneous precatalysts for as well as for the production of ethyl acetate.[2] A the disproportionation of aldehydes to the corre- number of compounds of the main group elements,[3] sponding carboxylic esters. Potassium compounds in transition metal[4] and rare earth[5] compounds, as well combination with 18-crown-6 ether could drastically as N-heterocyclic carbenes[6] are found to be active to increase the rate of reaction in a few cases. Alterna- catalyze this reaction. Traditional catalysts for this tively, efficient aldol condensations were found for process include mainly sodium[3a,b] and in particularly [3c–g] [3h] [3i] aldehydes possessing an enolizable methylene aluminium alkoxides. Boric acid, (i-Bu)2AlH, [3j,k] [3l] group at the a-position to the aldehyde functionali- alkaline earth metal amides, LiBr/Et3N, ty. The active species involved in this esterification NaH,[3b,m,n] Grignard reagents in combination with thi- using any of these alkali metal catalysts is expected olates,[3o] , selenide ions,[3p] transition metal complexes to be the metal alkoxide. Potassium compounds based on Fe,[4a,b] Ru,[4c–h] Rh,[4i–k] Os,[4l] Ir,[4m,n] Ni,[4o,p] were found to be much more efficient when com- Zr,[4q] Hf[4q] and lanthanide complexes,[5a–g] particularly pared to analogous sodium compounds and kinetic lanthanide amides,[5b–e] and organoactinide com- studies revealed the rate-determining step to be plexesACHTUNGTRENNUNG [5h,i] have also been employed for this reaction. a second order concerted hydride transfer from Quite recently, this disproportionation reaction be- a potassium hemiacetal species to another molecule tween two different selected aldehydes could be ac- of aldehyde. complished to a certain extent.[1f,4p] However, many of these auxiliaries are toxic, not commercially available Keywords: aldehydes; alkali metal compounds; or expensive, suffer from low reaction rates, require Claisen–Tishchenko reaction; crown ethers; esters high catalyst loadings, are air- and moisture-sensitive, undergo side reactions, react sluggishly and are ineffi- cient for heteroaromatic aldehydes, which are known to be difficult to disproportionate to esters.[3n,7] Recently, we have reported rhenium complexes as Simple, efficient and environmentally friendly trans- precatalysts for the hydrogenation of various olefins[8] formations are challenging goals in chemical synthe- and nitriles.[9] Later, when we started to study the ac- sis. The greatest achievements in these respects have tivity of one of these complexes in combination with evoked from the concept of catalysis. Featuring high t-BuOK for the catalytic hydrogenation of benzalde- atom economy, the dimerization or disproportionation hyde using H2, we came across the formation of of aldehydes to the corresponding carboxylic esters, benzyl benzoate as a by-product. Although the rheni- named Claisen–Tishchenko[1] reaction (Scheme 1), has um hydrides generating during this reaction are capa- ble of catalyzing this disproportionative esterification, we found that the simple, inexpensive and commer- cially available t-BuOK itself is an efficient catalyst for this reaction. Much to our surprise and to the best of knowledge, this reaction using t-BuOK as catalyst Scheme 1. The Claisen–Tishchenko reaction. has not been recognized or documented so far.
Adv. Synth. Catal. 2013, 355, 901 – 906 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 901
168 COMMUNICATIONS Kunjanpillai Rajesh and Heinz Berke
Table 1. Claisen–Tishchenko reaction catalyzed by t-BuOK.
Entry Product Ester [R][a] Cat. [mol%] Temp. [8C] TOF [1st h][b] [hÀ1] Time [h] Yield[c] [%] 1 phenyl 1 23 81 1.5 96 2 4-chlorophenyl 2 23 29 18 94 3 4-chlorophenyl 2 80 48 1 94 4 2-chlorophenyl 2 23 6 24 92 5 4-bromophenyl 2 23 42 1.5 93 6 4-fluorophenyl 5 80 17 1 80 7 4-methoxyphenyl 2 23 8 24 92 8 2-thienyl 2 23 7 24 88 9 2-furanyl 5 80 15 1.5 79
10[d] 5 80 14 1.5 77
[a] R for entries 1–9. [b] By GC/MS based on the consumption of aldehyde. [c] Isolated yield. [d] o-Phthalaldehyde was used as the starting material.
Table 2. Claisen–Tishchenko reaction catalyzed by alkali metal catalysts.
Entry Product Ester [R] Cat./[mol%]ACHTUNGTRENNUNG Temp. [8C] TOF [1st h][a] [hÀ1] Time [h] Yield[b] [%] 1 t-BuONa/5 23 6 9 86 ACHTUNGTRENNUNG [a] 2 Li[N(SiMe3)2]/5 23 – 90 <5 ACHTUNGTRENNUNG 3 Na[N(SiMe3)2]/5 23 4 60 85 ACHTUNGTRENNUNG 4K[N(SiMe3)2]/5 23 15 7 86 phenyl ACHTUNGTRENNUNG 5 Na[N(SiMe3)2]/2.5 60 22 12 88 ACHTUNGTRENNUNG 6K[N(SiMe3)2]/2.5 60 31 2 90 7 KH/1 23 84 1.3 97 8 KH/1 23 98 1 97[c] 9 4-fluorophenyl KH/5 80 17 1 84 ACHTUNGTRENNUNG 10 4-chlorophenyl K[N(SiMe3)2]/5 60 13 2 86 ACHTUNGTRENNUNG 11 4-bromophenyl K[N(SiMe3)2]/3 60 21 2 85 12 2-furanyl KH/5 80 16 1.2 83 13 tert-butyl KH/5 23 14 2 93[d] [a] By GC/MS based on the consumption of aldehyde. [b] Unless mentioned, all reactions were carried out in toluene; isolated yield. [c] The reaction was carried out without a solvent. [d] Solvent: benzene; yield by 1H NMR spectroscopy using mesitylene as internal standard.
Using 1 mol% of t-BuOK, benzaldehyde could be benzaldehyde, reaction with 5 mol% of t-BuONa at disproportionated to benzyl benzoate in 1.5 h at room room temperature showed a TOF of 6 hÀ1 in the first temperature in toluene with a TOF of 81 hÀ1 in the hour giving rise to 86% yield of benzyl benzoate in first hour giving 98% GC yield of benzyl benzoate 9 h (Table 2, entry 1). The scope of the t-BuOK-cata- along with 1% benzyl alcohol and 1% tert-butyl ben- lyzed reaction has been tested with various aromatic zoate. This gave an isolated yield of 96% of benzyl and heteroaromatic substrates (Table 1). The TOF of benzoate (Table 1, entry 1). To test whether the catal- the reaction could be increased by carrying it out at ysis is due to other metal contaminants, we tested elevated temperatures without affecting the yield 99.99% sublimed grade (trace metals basis) of t- (Table 1, entries 2 and 3). Unlike 2-thiophenecarbox- BuOK which showed almost the same results. t- aldehyde, a comparatively higher loading of 5 mol% BuONa was also found to be active for this reaction, of the catalyst had to be adopted at 808C in the case but was far less efficient. In the disproportionation of of 2-furfuraldehyde giving rise to a yield of 79% of
902 asc.wiley-vch.de 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Synth. Catal. 2013, 355, 901 – 906
169 Alkali Metal tert-Butoxides, Hydrides and Bis(trimethylsilyl)amides as Efficient Homogeneous Catalysts the ester in 1.5 h (Table 1, entries 8 and 9). An intra- molecular Claisen–Tishchenko reaction was observed when o-phthalaldehyde was used, which with 5 mol% of the catalyst at 808C gave a yield of 77% of the phthalide cyclic ester in 1.5 h (Table 1, entry 10). Furthermore, we tested alkali metal bis(trimethylsi- lyl)amides for this Claisen–Tishchenko reaction and found them to be efficient (Table 2). Much to our sur- prise, these readily available, simple alkali amides also catalyzed these reactions, but were as yet not documented in literature although alkaline earth am- idesACHTUNGTRENNUNG [3j,k] and lanthanide amides[5b] as catalysts for such reactions were reported recently. Heavier amides are usually prepared from the alkali metal amides.[10] The activities of the alkali amides for the Claisen–Tish- chenko reactions were found to increase from the ACHTUNGTRENNUNG almost inefficient Li[N(SiMe3)2] to the quite efficient Scheme 2. Proposed mechanism for the Claisen–Tishchenko ACHTUNGTRENNUNG reaction catalyzed by KH/t-BuOK/K[N(SiMeACHTUNGTRENNUNG ) ] with or K[N(SiMe3)2]. Although the reaction of benzaldehyde 3 2 with 5 mol% of catalyst loading gave the ester at without 18-crown-6 ether as a co-catalyst. room temperature, a lower loading of 2.5 mol% was sufficient to give even better yield of the ester at 608C (Table 2, entries 3–6). facts, a mechanism similar to one proposed for the The t-BuOK-catalyzed disproportionation of ben- lanthanide amide catalyses is suggested (Scheme 2).[5b] zaldehyde showed the formation of tert-butyl ben- It worth mentioning that the KH-catalyzed dispropor- zoate and benzyl alcohol, the latter is expected to be tionation of benzaldehyde could be carried out even formed when potassium benzyloxide is quenched with without any solvent giving rise to 97% yield of benzyl water. A kinetic study of this reaction with benzalde- benzoate (Table 2, entry 8). When compared to t- hyde showed a linear relationship between 1/[reac-ACHTUNGTRENNUNG BuOK as a catalyst and since the butyl ester cannot tant] and time indicating a second-order reaction with be formed, a higher amount of the catalyst appears to respect to the reactant, benzaldehyde (Figure 1). The be present causing more of the product in the KH- metal alkoxide is assumed to be the active species, for catalyzed reaction (cf. Table 1, entries 6 and 9, respec- which further evidence could be obtained from the tively, to Table 2, entries 9 and 12). Using 5 mol% of ability of KH to catalyze this reaction with the same KH, the aliphatic tertiary aldehyde, trimethyl acetal- order, but appeared in comparison to be a little faster dehyde could be disproportionated to the correspond- which is attributed to the direct formation of the po- ing ester in 93% yield (Table 2, entry 13). tassium alkoxide (Table 2, entry 7). Based on these However, in the case of t-BuONa similar observa- tions were made as in the case of t-BuOK except that the reaction revealed a first order kinetic (Figure 2). Since tert-butyl benzoate is initially formed quantita- tively with respect to the precatalyst t-BuONa, the formation of the sodium benzyloxide could not be the rate-determining step. So, it is assumed that either the insertion of aldehyde into the sodium alkoxide (B,Na instead of K) or the non- concerted hydride transfer from the formed sodium hemiacetal species (C, Na in- stead of K) to another molecule of aldehyde would be the rate-determining step. If the hydride transfer occurs in a concerted manner (D, Na instead of K), then the former would be the rate-determining step. However, the latter pathway, which would involve a b-hydride elimination from the sodium hemiacetal species to form NaH, seems not to be plausible. Roesky and co-workers have rationalized a mecha- nism involving a concerted hydride transfer for homo- Figure 1. Linear plot of 1/[benzaldehyde] vs. time in the t- leptic lanthanide amide-catalyzed Claisen–Tishchenko [5b] BuOK-catalyzed Claisen–Tishchenko reaction indicating reaction, like the one suggested here for t-BuOK. a second order kinetics. A similar mechanism can be proposed for the
Adv. Synth. Catal. 2013, 355, 901 – 906 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim asc.wiley-vch.de 903
170 COMMUNICATIONS Kunjanpillai Rajesh and Heinz Berke
ed by Hill and co-workers in the case of alkaline earth metal amides catalyzing the Claisen–Tishchenko reaction.[3j] In order to improve the rate of this disproportiona- tion reaction catalyzed by potassium compounds, we added 18-crown-6 (1 equiv. with respect to the cata- lyst) (Table 3). The reaction rates were drastically im- proved in a few cases. With 1 mol% of t-BuOK and 18-crown-6 ether, the disproportionation of benzalde- hyde in benzene was completed in 10 min at room temperature giving rise to 96% yield of benzyl ben- zoate with a TOF of 582 hÀ1 (Table 3, entry 1). Under the same reaction conditions and with the same load- ings, KH showed a TOF of 1176 hÀ1 completing the reaction in 5 min with 97% yield of benzyl benzoate (Table 3, entry 2). With loadings of 2 mol% ACHTUNGTRENNUNG Figure 2. Linear plot of ln[benzaldehyde] vs. time in the t- K[N(SiMe3)2] and 18-crown-6 ether, the reaction À1 BuONa-catalyzed Claisen–Tishchenko reaction indicating showed a TOF of 6 h giving rise to 91% yield of a first order kinetics. benzyl benzoate in 8 h. This strategy was adopted for the disproportionation of a few more aromatic alde- hydes (Table 3, entries 3–7). However, when com- pared, addition of this crown ether decreased the effi- ACHTUNGTRENNUNG K[N(SiMe3)2]-catalyzed disproportionation. At 30 min ciency in the case of 2-furfuraldehyde and 2-thienyl- of the reaction of benzaldehyde with 0.5 equiv. of carboxaldehyde and was almost inactive for the dis- ACHTUNGTRENNUNG K[N(SiMe3)2], GC/MS analysis showed benzyl ben- proportionation of 4-fluorobenzaldehyde. Since the zoate, benzyl alcohol – which is expected to be active species involved is expected to be the 18- formed when potassium benzyloxide is quenched with crown-6 ether potassium benzyloxide when any of water, the hydrosilylated product benzyl trimethylsilyl these potassium compounds are used, we studied the ether and traces of other unidentified products. How- kinetics of this reaction with benzaldehyde using the ACHTUNGTRENNUNG ever, the major oxidized species could not be detect- K[N(SiMe3)2]/18-crown-6 ether system and found it to ed. Some of the above observations were also report- be second order with respect to benzaldehyde, like
Table 3. Claisen–Tishchenko reaction catalyzed by potassium compounds along with 1 equiv. of 18-crown-6 ether with re- spect to the catalysts.
Entry Product Ester [R] Cat./[mol%]ACHTUNGTRENNUNG TOF[a] [hÀ1] Time [h] Yield[b] [%] 1 t-BuOK/1 582 0.17 96 2 phenyl KH/1 1176 0.083 97 ACHTUNGTRENNUNG 3K[N(SiMe3)2]/2 6 8 91 4 4-chlorophenyl KH/2 32 1.5 92 5 2-chlorophenyl KH/2 144 0.33 95 6 4-bromophenyl KH/1 291 0.33 95[c] 7 4-methoxyphenyl KH/1 39 2.5 97 8 tert-butyl KH/5 74 0.25 93[d] 9 cyclohexyl KH/5 – 10 45[c,e] 10 n-pentyl KH/3 – 3 –[f] [a] TOF by GC/MS based on the consumption of the aldehyde. [b] Unless mentioned, solvent: benzene; temperature 238C; isolated yield. [c] Solvent: toluene [d] Yield by 1H NMR spectroscopy using mesitylene as internal standard. [e] Temperature 808C. [f] Aldol condensation products in 90% (E/Z=97/3); traces of other isomers were also formed; yield by GC/MS based on the consumption of aldehyde.
904 asc.wiley-vch.de 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Synth. Catal. 2013, 355, 901 – 906
171 Alkali Metal tert-Butoxides, Hydrides and Bis(trimethylsilyl)amides as Efficient Homogeneous Catalysts
vide the product with acceptable rates and yields. Higher efficiencies of the catalysts are expected to be achieved by utilizing the opportunity of ligand sphere tuning, particularly with transition metals as well as lanthanides and actinides, which has acquired suffi- cient interest in recent times and as mentioned, we are also involved in developing this transformation using suitable rhenium complexes. This would also impart help to overcome the limitations of any pri- mary aliphatic aldehydes, as well as to effect cross- Claisen–Tishchenko disproportionation.
Experimental Section
Alkali Metal Compounds-Catalyzed Claisen– Tishchenko Reaction Figure 3. Linear plot of 1/[benzaldehyde] vs. time in the t-BuOK (10 mg, 0.089 mmol) was charged in a 10-mL glass ACHTUNGTRENNUNG K[N(SiMe3)2]/18-crown-6 ether-catalyzed Claisen–Tishchen- vial (for room temperature reactions) or a 25-mL Young ko reaction indicating a second order kinetics. Schlenk tube (for heating reactions). Toluene (1 mL) was added to it. To this, benzaldehyde (945.8 mg, 8.9 mmol) was added followed by another portion of toluene (1 mL). The the one without 18-crown-6 ether, indicating a similar vessel was closed and the mixture was stirred (in the glove mechanism (Figure 3). We suppose that the increased box itself for room temperature reactions or kept outside in activity by adding the crown ether is caused by an in- an oil bath for heating reactions). The reaction was moni- creased solubility of the potassium (crown ether) salt. tored by GC/MS for which the samples were taken in the Then, we tested this strategy also for aliphatic pri- glove box, quenched immediately with water outside. When the reaction was completed, the mass was concentrated to mary, secondary and tertiary aldehydes as reaction dryness. It was purified by silica gel flash column chroma- substrates. The aliphatic tertiary aldehyde, trimethyla- tography (eluent: hexane/ethyl acetate) to afford benzyl cetaldehyde could be disproportionated to the corre- benzoate as a pale yellow liquid; yield: 907.1 mg sponding ester in 93% yield within 15 min at room (4.274 mmol, 96%). ACHTUNGTRENNUNG ACHTUNGTRENNUNG temperature using KH and 18-crown-6 ether loadings The KH, t-BuONa, K[N(SiMe3)2] and Na[N(SiMe3)2] cata- of 5 mol% (Table 3, entry 8). Though the reaction was lyzed Claisen–Tishchenko reactions were also performed in not uniform, addition of 5 mol% of KH and 18- a similar manner. crown-6 ether at a temperature of 808C was adopted for the disproportionation of the aliphatic aldehyde, Potassium Compounds/18-Crown-6 Ether-Catalyzed cyclohexanecarboxaldehyde, which gave 45% yield of Claisen–Tishchenko Reaction the desired ester in 10 h (Table 3, entry 9). However, KH (3 mg, 0.0748 mmol) and 18-crown-6 ether (19.75 mg, aldehydes possessing a methylene group at the a-posi- 0.0748 mmol) were charged in a Young NMR tube. Ben- tion to the aldehyde functionality, hexanal for in- zene-d6 (0.3 mL) was added to it. To this, benzaldehyde stance, revealed under the conditions of the addition (793.7 mg, 7.48 mmol) was added followed by another por- of 3 mol% of KH and 18-crown-6 ether at room tem- tion of benzene-d6 (0.3 mL). The tube was closed and the re- perature, formation of aldol condensation products in action mass was shaken well. The reaction was monitored 1 >90% yield (E/Z=97/3) (Table 3, entry 10). by H NMR spectroscopy for the absence of benzaldehyde. In summary, the simple, cheap, shelf-available t- When the reaction was completed, the mass was concentrat- BuOK and KH, as well as t-BuONa are efficient cata- ed to dryness. It was purified by silica gel flash column chro- matography (eluent: hexane/ethyl acetate) to afford benzyl lysts for the Claisen–Tishchenko disproportionation benzoate as a pale yellow liquid; yield: 767.35 mg of aromatic and even heteroaromatic aldehydes, as (3.615 mmol, 97%). well as of aliphatic secondary and tertiary aldehydes. ACHTUNGTRENNUNG t-BuOK/18-crown-6 ether and K[N(SiMe3)2]/18-crown-6 Alternatively, aldol condensation products were ether catalyzed Claisen–Tishchenko reactions were also per- formed when primary aldehydes were used. Addition formed in a similar manner. of a catalytic quantity of 18-crown-6 ether could in- crease the rate of reaction in some cases. The simple potassium and sodium metal amides, although less ef- Acknowledgements ficient for the disproportionation reaction when com- pared to the corresponding homoleptic lanthanide Funding from the Swiss National Science Foundation and the amides, as well as alkaline earth amides, could pro- University of Zurich is gratefully acknowledged.
Adv. Synth. Catal. 2013, 355, 901 – 906 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim asc.wiley-vch.de 905
172 COMMUNICATIONS Kunjanpillai Rajesh and Heinz Berke
References 2004, 224, 105–109; h) M.-O. Simon, S. Darses, Adv. Synth. Catal. 2010, 352, 305–308; i) M. Massoui, D. [1] a) L. Claisen, Ber. Dtsch. Chem. Ges. 1887, 20, 646– BeaupØre, L. Nadjo, R. Uzan, J. Organomet. Chem. 650; b) W. Tischtschenko, Chem. Zentralbl. 1906, 77,I, 1983, 259, 345–348; j) C. Tejel, M. A. Ciriano V. Passar- 1309–1311; c) T. Seki, T. Nakajo, M. Onaka, Chem. elli, Chem. Eur. J. 2011, 17, 91–95; k) S. H. Bergens, Lett. 2006, 35, 824–829; d) M. H. S. A. Hamid, P. A. D. P. Fairlie, B. Bosnich, Organometallics 1990, 9, 566– Slatford, J. M. J. Williams, Adv. Synth. Catal. 2007, 349, 571; l) P. Barrio, M. A. Esteruelas, E. Onate, Organo- 1555–1575; e) K. Ekoue-Kovi, C. Wolf, Chem. Eur. J. metallics 2004, 23, 1340–1348; m) T. Suzuki, T; 2008, 14, 6302–6315; f) W. I. Dzik, L. J. Gooßen, Yamada, T. Matsuo, K. Watanabe, T. Katoh, Synlett Angew. Chem. 2011, 123, 11241–11243; Angew. Chem. 2005, 1450–1452; n) T. Suzuki, T. Yamada, K. Wata- Int. Ed. 2011, 50, 11047–11049. nabe, T. Katoh, Bioorg. Med. Chem. Lett. 2005, 15, th [2] a) Ullmann s Encyclopadia of Industrial Chemistry,6 2583–2585; o) S. Ogoshi, Y. Hoshimoto, M. Ohashi, edn., Wiley-VCH, Weinheim, 2002. Chem. Commun. 2010, 46, 3354–3356; p) Y. Hoshimoto, [3] a) O. Kamm,W. F. Kamm, Org. Synth. Coll. Vol. 1, M. Ohashi, S. Ogoshi, J. Am. Chem. Soc. 2011, 133, 1941,104; b) F. W. Swamer, C. R. Hauser, J. Am. Chem. 4668–4671; q) K.-I. Morita, Y. Nishiyama, Y. Ishii, Or- Soc. 1946, 68, 2647–2649; c) W. C. Child, H. Atkins, J. ganometallics 1993, 12, 3748–3752. Am. Chem. Soc.1923, 45, 3013–3023; d) Y. Ogata, A. [5] a) S. Onozawa, T. Sakakura, M. Tanaka, M. Shiro, Tet- Kawasaki, Tetrahedron 1969, 25, 929–935; e) T. Ooi, T. rahedron 1996, 52, 4291–4302; b) H. Berberich, P. W. Miura, K. Takaya, Tetrahedron Lett. 1999, 40, 7695– Roesky, Angew. Chem. 1998, 110, 1618–1620; Angew. 7698; f) I. Simpura, V. Nevalainen, Tetrahedron 2001, Chem. Int. Ed. 1998, 37, 1569–1571; c) G. B. Deacon, 57, 9867–9872; g) T. Ooi, K. Ohmatsu, K. Sasaki, T. A. Gitlits, P. W. Roesky, M. R. Bürgstein, K. C. Lim, Miura, K. Maruoka, Tetrahedron Lett. 2003, 44, 3191– B. W. Skelton, A. H. White, Chem. Eur. J. 2001, 7, 127– 3193; h) P. R. Stupp, J. Org. Chem. 1973, 38, 1433– 138; d) M. R. Burgstein, H. Berberich, P. W. Roesky, 1434; i) Y.-S. Hon, Y.-C. Wong, C.-P. Chang, C.-H. Chem. Eur. J. 2001, 7, 3078–3085; e) A. Zuyls, P. W. Hsieh, Tetrahedron 2007, 63, 11325–11340; j) M. R. Roesky, G. B. Deacon, K. Konstas, P. C. Junk, Eur. J. Crimmin, A. G. M. Barrett, M. S. Hill, P. A. Procopiou, Org. Chem. 2008, 693–697; f) J.-L. Hsu, J.-M. Fang, J. Org. Lett. 2007, 9, 331–333; k) B. M. Day, N. E. Mans- field, M. P. Coles, P. B. Hitchcock, Chem. Commun. Org. Chem. 2001, 66, 8573–8584; g) A. Michrowska, B. 2011, 47, 4995–4997; l) M. M. Mojtahedi, E. Akbarza- List, Nature Chem. 2009, 1, 225–228; h) T. Andrea, E. deh, R. Sharifi, M. S. Abaee, Org. Lett. 2007, 9, 2791– Barnea, M. S. Eisen, J. Am. Chem. Soc. 2008, 130, 2793; m) D. C. Waddell, J. Mack, Green Chem. 2009, 2454–2455; i) M. Sharma, T. Andrea, N. J. Brookes, 11, 79–82; n) T. Werner, J. Koch, Eur. J. Org. Chem. B. F. Yates, M. S. Eisen, J. Am. Chem. Soc. 2011, 133, 2010, 6904–6907; o) L. Cronin, F. Manoni, C. J. O 1341–1356. Connor, S. J. Connon, Angew. Chem. 2010, 122, 3109– [6] A. Chan, K. A. Scheidt, J. Am. Chem. Soc. 2006, 128, 3112; Angew. Chem. Int. Ed. 2010, 49, 3045–3048; p) S. 4558–4559. P Curran, S. J. Connon, Org. Lett. 2012, 14, 1074–1077. [7] T. Seki, K. Akutsu, H. Hattori, Chem. Commun. 2001, [4] a) M. Yamashita, Y. Watanabe, T.-A. Mitsudo, Y. Take- 1000–1001. gami, Bull. Chem. Soc. Jpn. 1976, 49, 3597–3600; b) M. [8] a) B. Dudle, K. Rajesh, O. Blacque, H. Berke, J. Am. Yamashita, T. Ohishi, Appl. Organomet. Chem. 1993, 7, Chem. Soc. 2011, 133, 8168–8178. 357–361; c) H. Horino, T. Ito, A. Yamamoto, Chem. [9] K. Rajesh, B. Dudle, O. Blacque, H. Berke, Adv. Synth. Lett. 1978, 7, 17–20; d) T. Ito, H. Horino, Y. Koshiro, Catal. 2011, 353, 1479–1484. A. Yamamoto, Bull. Chem. Soc. Jpn. 1982, 55, 504–512; [10] a) D. C. Bradley, J. S. Ghorta, F. A. Hart, J. Chem. Soc. e) N. Menashe, Y. Shvo, Organometallics 1991, 10, Dalton Trans. 1973, 1021–1023; b) M. H. Chisholm, J. 3885–3891; f) V. V. Grushin, H. Alper, J. Org. Chem. Gallucci, K. Phomphari, Chem. Commun. 2003, 48–49; 1991, 56, 5159–5161; g) A. Sorkau, K. Schwarzer, C. c) M. H. Chisholm, J. Gallucci, K. Phomphrai, Inorg. Wagner, E. Poetsch, D. Steinborn, J. Mol. Catal. A Chem. 2004, 43, 6717–6725.
906 asc.wiley-vch.de 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Synth. Catal. 2013, 355, 901 – 906
173 Supporting Information
Alkali Metal tert-Butoxides, Hydrides and Bis(trimethylsilyl)amides as Efficient Homogeneous Catalysts for Claisen-Tishchenko Reaction†
Kunjanpillai Rajesh and Heinz Berke*
Institute of Inorganic Chemistry University of Zürich, Winterthurerstrasse 190, CH-8057, Zürich, Switzerland; Fax: 41-1-635-6802; E-mail: [email protected]
General Information
All manipulations were carried out under an atmosphere of dry nitrogen using standard Schlenk techniques or in a glove box (M. Braun 150B-G-II) filled with dry nitrogen. Solvents were freshly distilled under N2 by employing standard procedures. NaH, t-BuONa, t-BuOK, t-BuOK (sublimed grade, 99.99% trace metals basis), NaN[(SiMe3)2] and K[N(SiMe3)2] were purchased from Aldrich. KH in mineral oil was purchased from Acros Organics. Both NaH and KH were washed with hexane in a glove box and the dry powders were stored in the glove box. Redistilled benzaldehyde purchased from Aldrich chemical company and all other aldehydes purchased either from Aldrich or ABCR chemical companies were used as such. NMR analyses were carried out on a Varian Gemini 300 spectrometer. GC/MS analyses were carried out on a Varian Saturn 2000 GC/MS spectrometer.
Kinetic Studies
Kinetics of the t-BuOK and t-BuONa reactions were carried out in toluene in a Young Schlenk where as the K[N(SiMe3)2]/18-crown-6 ether catalyzed reaction was carried out in benzene in a Young NMR tube.
Purification of Products
All the products were obtained by purification using silica gel column chromatography (Eluent: Hexane/Ethyl acetate) except cyclohexylmethyl cyclohexanecarboxylate which was isolated by distillation.
174 Characterization Data
Benzyl benzoate
O
O
Table 1, entry 1: Using 1 mol% of t-BuOK (10 mg, 0.089 mmol) in toluene (2 mL) at rt, the aldehyde (945.8 mg, 8.9 mmol) was dimerized to yield 96% of the ester (908 mg, 4.278 mmol). Table 2, entry 7: Using 1 mol% of KH (5 mg, 0.125 mmol) in toluene at rt (2.5 mL), the aldehyde (1.323 g, 12.5 mmol) was dimerized to yield 97% of the ester (1.283 g, 6.045 mmol). Table 2, entry 8: Using 1 mol% of KH (5 mg, 0.125 mmol) in neat conditions at rt (2.5 mL), the aldehyde (1.323 g, 12.5 mmol) was dimerized to yield 97% of the ester (1.283 g, 6.045 mmol). Table 2, entry 1: Using 5 mol% of t-BuONa (10 mg, 0.104 mmol) in toluene (0.8 mL) at rt, the aldehyde (220.9 mg, 2.08 mmol) was dimerized to yield 86% of the ester (189.9 mg, 0.895 mmol). Table 2, entry 3: Using 5 mol% of Na[N(SiMe3)2] (10 mg, 0.0545 mmol) in toluene (0.6 mL) at rt, the aldehyde (115.7 mg, 1.09 mmol) was dimerized to yield 85% of the ester (98.35 mg, 0.463 mmol). Table 2, entry 4: Using 5 mol% of K[N(SiMe3)2] (10 mg, 0.05 mmol) in toluene (0.6 mL) at rt, the aldehyde (106.4 mg, 1.0 mmol) was dimerized to yield 86% of the ester (91.50 mg, 0.431 mmol). Table 2, entry 5: Using 2.5 mol% of Na[N(SiMe3)2] (10 mg, 0.0545 mmol) in toluene (1 mL) at 60 °C, the aldehyde (231.5 mg, 2.18 mmol) was dimerized to yield 88% of the ester (203.7 mg, 0.961 mmol). Table 2, entry 6: Using 2.5 mol% of K[N(SiMe3)2] (10 mg, 0.05 mmol) in toluene (1 mL) and at 60 °C, the aldehyde (212.79 mg, 2.0 mmol) was dimerized to yield 90% of the ester (191.5 mg, 0.902 mmol). Table 3, entry 1: Using 1 mol% each of t-BuOK (5 mg, 0.0446 mmol) and 18-crown-6 ether (11.77 mg, 0.0446 mmol) in benzene (0.4 mL) at rt, the aldehyde (472.9 mg, 4.456 mmol) was dimerized to yield 96% of the ester (457.8 mg, 2.157 mmol). Table 3, entry 2: Using 1 mol% each of KH (3 mg, 0.0748 mmol) and 18-crown-6 ether (19.75 mg, 0.0748 mmol) in benzene (0.6 mL) at rt, the aldehyde (793.7 mg, 7.48 mmol) was dimerized to yield 97% of the ester (769.89 mg, 3.627 mmol). Table 3, entry 3: Using 1 mol% each of K[N(SiMe3)2] (10 mg, 0.05 mmol) and 18-crown-6 ether (13.24 mg, 0.05 mmol) in benzene (0.4 mL) at rt, the aldehyde (266.0 mg, 2.51 mmol) was dimerized to yield 91% of the ester (242.05 mg, 1.14 mmol).
1 H NMR (300 MHz, CDCl3): δ 5.41 (s, 2H), 7.38-7.51 (m, 7H), 7.57-7.62 (m, 1H), 8.11-8.14 (m, 13 2H); C NMR (75 MHz, CDCl3): δ 66.6, 128.1, 128.2, 128.3, 128.5, 129.6, 130.1, 133.0, 136.0, 166.3.
175 4-Methoxybenzyl 4-methoxybenzoate
O
O MeO OMe
Table 1, entry 7: Using 2 mol% of t-BuOK (10 mg, 0.089 mmol) in toluene (2 mL) at rt, the aldehyde (606.5 mg, 4.455 mmol) was dimerized to yield 92% of the ester (558.0 mg, 2.049 mmol). Table 3, entry 7: Using 1 mol% each of KH (3 mg, 0.0748 mmol) and 18-crown-6 ether (19.75 mg, 0.0748 mmol) in benzene (0.6 mL) at rt, the aldehyde (1.018 g, 7.48 mmol) was dimerized to yield 97% of the ester (987.8 mg, 3.627 mmol).
1 H NMR (300 MHz, CDCl3): δ 3.82 (s, 3H), 3.85 (s, 3H), 5.29 (s, 2H), 6.90-6.94 (m, 4H), 7.38- 13 7.41 (m, 2H), 8.02-8.05 (m, 2H); C NMR (75 MHz, CDCl3): δ 55.3, 55.4, 66.2, 113.6, 113.9, 122.7, 128.4, 130.0, 131.7, 159.6, 163.4, 166.2.
4-Fluorobenzyl 4-fluorobenzoate
O
O
F F
Table 1, entry 6: Using 5 mol% of t-BuOK (10 mg, 0.089 mmol) in toluene (0.6 mL) at 80 °C, the aldehyde (221.2 mg, 1.782 mmol) was dimerized to yield 80% of the ester (177.0 mg, 0.713 mmol). Table 2, entry 9: Using 5 mol% of KH (5 mg, 0.125 mmol) in toluene at 80 °C (0.6 mL), the aldehyde (0.309 g, 2.493 mmol) was dimerized to yield 84% of the ester (259.6 g, 1.046 mmol).
1 H NMR (300 MHz, CDCl3): δ 5.33 (s, 2H), 7.06-7.15 (m, 4H), 7.41-7.46 (m, 2H), 8.06-8.11 (m, 13 2H); C NMR (75 MHz, CDCl3): δ 66.2, 115.6 (d, J = 21.7 Hz), 126.4 (d, J = 3.1 Hz), 130.3 (d, J = 8.2 Hz), 131.9 (d, J = 3.1 Hz), 132.3 (d, J = 9.3 Hz), 161.1, 164.3 (d, J = 14.3 Hz), 165.4, 167.6. 4-Chlorobenzyl 4-chlorobenzoate
O
O
Cl Cl
176 Table 1, entry 2: Using 2 mol% of t-BuOK (10 mg, 0.089 mmol) in toluene (2 mL) at rt, the aldehyde (626.6 mg, 4.456 mmol) was dimerized to yield 94% of the ester (589.0 mg, 2.095 mmol). Table 2, entry 10: Using 5 mol% of K[N(SiMe3)2] (10 mg, 0.05 mmol) in toluene (0.8 mL) at 60 °C, the aldehyde (140.94 mg, 1.003 mmol) was dimerized to yield 86% of the ester (121.2 mg, 0.431 mmol) Table 3, entry 4: Using 2 mol% each of KH (3 mg, 0.0748 mmol) and 18-crown-6 ether (19.75 mg, 0.0748 mmol) in benzene (0.6 mL) at rt, the aldehyde (525.7 mg, 3.74 mmol) was dimerized to yield 92% of the ester (483.64 mg, 1.72 mmol).
1 H NMR (300 MHz, CDCl3): δ 5.33 (s, 2H), 7.38-7.46 (m, 6H), 7.40-7.45 (m, 2H), 7.99-8.02 (m, 13 2H); C NMR (75 MHz, CDCl3): δ 66.1, 128.3, 128.8, 128.8, 129.6, 131.1, 134.3, 134.3, 139.6, 165.4
4-Bromobenzyl 4-bromobenzoate
O
O
Br Br
Table 1, entry 5: Using 2 mol% of t-BuOK (10 mg, 0.089 mmol) in toluene (2 mL) at rt, the aldehyde (824.4 mg, 4.456 mmol) was dimerized to yield 93% of the ester (766.7 mg, 2.072 mmol). Table 2, entry 11: Using 3 mol% of K[N(SiMe3)2] (10 mg, 0.05 mmol) in toluene (0.8 mL) and at 60 °C, the aldehyde (309.1 mg, 1.671 mmol) was dimerized to yield 85% of the ester (262.74 mg, 0.710 mmol) Table 3, entry 6: Using 1 mol% each of KH (3 mg, 0.0748 mmol) and 18-crown-6 ether (19.75 mg, 0.0748 mmol) in toluene (2 mL) at rt, the aldehyde (1.384 g, 7.48 mmol) was dimerized to yield 95% of the ester (1.315 mg, 3.55 mmol).
1 H NMR (300 MHz, CDCl3): δ 5.31 (s, 2H), 7.27-7.34 (m, 2H), 7.52-7.61 (m, 4H), 7.91-7.94 (m, 13 2H); C NMR (75 MHz, CDCl3): δ 66.2, 122.4, 128.3, 128.8, 129.9, 131.2, 131.8, 131.8, 134.7, 165.5.
2-Chlorobenzyl 2-chlorobenzoate
O
O
Cl Cl
177 Table 1, entry 4: Using 2 mol% of t-BuOK (10 mg, 0.089 mmol) in toluene (2 mL) at rt, the aldehyde (626.6 mg, 4.456 mmol) was dimerized to yield 92% of the ester (576.5 mg, 2.05 mmol). Table 3, entry 5: Using 2 mol% each of KH (3 mg, 0.0748 mmol) and 18-crown-6 ether (19.75 mg, 0.0748 mmol) in benzene (0.6 mL) at rt, the aldehyde (525.7 mg, 3.74 mmol) was dimerized to yield 95% of the ester (499.42 mg, 1.776 mmol).
1 H NMR (300 MHz, CDCl3): δ 5.49 (s, 2H), 7.27-7.34 (m, 3H), 7.39-7.48 (m, 3H), 7.53-7.57 (m, 13 1H), 7.88-7.91 (m, 1H); C NMR (75 MHz, CDCl3): δ 64.6, 126.6, 126.9, 129.7, 129.7, 130.1, 131.1, 131.6, 132.7, 133.2, 133.8, 133.9, 165.1.
Isobenzofuran-1(3H)-one
O
O
Table 1, entry 10: Using 5 mol% of t-BuOK (10 mg, 0.089 mmol) in toluene (2 mL) at 80 °C, the aldehyde (239.07 mg, 1.782 mmol) was dimerized to yield 77% of the ester (184.08 mg, 1.372 mmol).
1 H NMR (300 MHz, CDCl3): δ 5.31 (s, 2H), 7.49-7.54 (m, 2H), 7.66-7.71 (m, 1H), 7.86-7.89 (m, 13 1H); C NMR (75 MHz, CDCl3): δ 69.9, 122.3, 125.4, 125.6, 129.1, 134.1, 146.7, 171.2.
Furan-2-ylmethyl furan-2-carboxylate
O
O O O
Table 1, entry 9: Using 5 mol% of t-BuOK (10 mg, 0.089 mmol) in toluene (2 mL) at 80 °C, the aldehyde (171.25 mg, 1.782 mmol) was dimerized to yield 79% of the ester (135.29 mg, 0.704 mmol). Table 2, entry 12: Using 5 mol% of KH (5 mg, 0.125 mmol) in toluene at 80 °C (0.6 mL), the aldehyde (239.54 mg, 2.493 mmol) was dimerized to yield 83% of the ester (198.82 g, 1.035 mmol).
1 H NMR (300 MHz, CDCl3): δ 5.29 (s, 2H), 6.36-6.38 (m, 1H), 6.48-6.50 (m, 2H), 7.19-7.20 (m, 13 1H), 7.43-7.44 (m, 1H), 7.56-7.57 (m, 1H); C NMR (75 MHz, CDCl3): δ 58.2, 110.6, 111.2, 111.8, 118.4, 143.4, 144.2, 146.5, 149.0, 158.2.
Thiophen-2-ylmethyl thiophene-2-carboxylate
S
O S
O
178 Table 1, entry 8: Using 2 mol% of t-BuOK (10 mg, 0.089 mmol) in toluene (2 mL) at rt, the aldehyde (499.7 mg, 4.456 mmol) was dimerized to yield 88% of the ester (439.76 mg, 1.961 mmol).
1 H-NMR (300 MHz, CDCl3): δ 5.50 (s, 2H), 7.01 (dd, J = 3.6, 5.1 Hz, 1H), 7.09 (dd, J = 3.9, 5.1 Hz, 1H), 7.18 (dd, J = 0.6, 3.3 Hz, 1H), 7.35 (dd, J = 1.2, 5.1 Hz, 1H), 7.56, (dd, J = 1.2, 4.8 Hz, 13 1H), 7.83 (dd, J = 1.2, 3.9 Hz, 1H); C NMR (75 MHz, CDCl3) δ 61.1, 126.8, 127.0, 127.8, 128.4, 132.7, 133.4, 133.8, 137.8, 161.9.
Cyclohexylmethyl cyclohexanecarboxylate
O
O
Table 3, entry 9: Using 5 mol% each of KH (5 mg, 0.125 mmol) and 18-crown-6 ether (32.92 mg, 0.125 mmol) in toluene (0.6 mL) at rt, the aldehyde (279.66 mg, 2.49 mmol) was dimerized to yield 45% of the ester by distillation (125.85 mg, 0.561 mmol).
1 H NMR (CDCl3, 400 MHz) 0.90-1.02 (m, 2H), 1.19-1.49 (m, 8H), 1.61-1.73 (m, 9H), 1.87-1.92 13 (m, 2H), 2.29 (tt, J = 3.3, 11.1 Hz, 9H), 3.86 (d, J = 6.3 Hz 2H); C NMR (CDCl3, 75 MHz) 25.4, 25.7, 25.7, 26.3, 29.0, 29.6, 37.1, 43.2, 69.2, 176.1
2,2-dimethylpropyl 2,2-dimethylpropanoate O O
Due to its comparatively low boiling point, this compound was directly analyzed by 1H NMR spectroscopy using mesitylene as internal standard.
Table 2, entry 13: Using 5 mol% of KH (5 mg, 0.125 mmol in benzene (0.6 mL) at rt, the aldehyde (214.75 mg, 2.49 mmol) was dimerized to yield 93% of the ester .
Table 3, entry 8: Using 5 mol% each of KH (5 mg, 0.125 mmol) and 18-crown-6 ether (32.92 mg, 0.125 mmol) in benzene (0.6 mL) at rt, the aldehyde (214.75 mg, 2.49 mmol) was dimerized to yield 93% of the ester
1 H NMR (300 MHz, CDCl3, mesitylene): δ 0.83 (s, 9H), 1.18 (s, 9H), 3.71 (s, 2H).
References
1. T. Werner, J. Koch, Eur. J. Org. Chem. 2010, 6904-6907. 2. M. R. Crimmin, A. G. M. Barrett, M. S. Hill, P. A. Procopiou, Org. Lett. 2007, 9, 331-333;
179 1H and 13C NMR Spectra
O
O
(Table 1, entry 1)
O
O
(Table 1, entry 1)
180 O
O
MeO (Table 1, entry 7) OMe
O
O
MeO (Table 1, entry 7) OMe
181 O
O
F (Table 1, entry 6) F
O
O
F (Table 1, entry 6) F
182 O
O
Cl (Table 1, entry 2) Cl
O
O
Cl (Table 1, entry 2) Cl
183 O
O
Br (Table 1, entry 5) Br
O
O
Br (Table 1, entry 5) Br
184 O
O
Cl Cl (Table 1, entry 4)
O
O
Cl Cl (Table 1, entry 4)
185 O
O
(Table 1, entry 10)
O
O
(Table 1, entry 10)
186
O
O O
O (Table 1, entry 9)
O
O O
O (Table 1, entry 9)
187 O
S O
S (Table 1, entry 8)
O
S O
S (Table 1, entry 8)
188
O
O
(Table 3, entry 9)
O
O
(Table 3, entry 9)
189 O
O
(Table 2, entry 13) Crude spectra - Mesitylene as internal standard
190 Kinetics of t-BuOK/Benzaldehyde
Benzaldehyde Benzyl benzoate
191 Kinetics of t-BuOK/Benzaldehyde continued...
192 Kinetics of t-BuONa/Benzaldehyde
Benzyl alcohol t -Butyl benzoate BHT (Stabilizer from ether)
193 t-BuOK/Benzaldehyde showing benzyl alcohol and t-butyl benzoate
BHT (Stabilizer from ether)
Benzaldehyde
Benzyl benzoate
Benzyl alcohol
t-Butyl benzoate
194 t-BuONa/Benzaldehyde showing benzyl alcohol and t-butyl benzoate
Benzaldehyde
Benzyl benzoate
Benzyl alcohol
t-Butyl benzoate
195 At 30 min of the reaction of benzaldehyde with K[N(SiMe3)2]in toluene
Benzaldehyde
Benzyl alcohol
Benzyl trimethylsilyl ether
Benzyl benzoate
196 Summary
Summary
Novel class of large bite angle diphosphine nitrosyl rhenium complexes were prepared and their catalytic capabilities for hydrogenations and transfer hydrogenations of a variety of functionalities and hydrosilylations of nitriles, as well as Claisen-Tishchenko disproportionative esterification of aldehydes were realized. Alkali metal compounds catalyzed Claisen-Tishchenko Reactions were also discovered. Summarized schemes of these transformations are shown below.
Alkanes Chapter 2 Amines Chapter 3 N-silylaldimines 2 Chapter 9 H y d r o s i Amines N ly i l Chapter 4 tr a i ti Amines le o 2 s n /H Chapter 8 Tr s ans e f er l H ri yd Alkenes and Alkynes/H N ro it H2 itri gena nes/ les tion N Imi n Re tio CO na 2/H oge 2 or E ydr s A t3SiH H ne l r i C d sf e Im e Chapter 5 n d O h Tra an a y s n ne 2 d MeOH s d , eto e e K b s d E o y i
c h s r
a Alcohols/Amines e t K e d r e l r b t s o Chapter 7 A o / n n H e a s 2 / t H e
s 2
Na Aldehydes Esters Alcohols K Alcohols, Chapter 6 Chapter 7 and 11 Formates Chapter 6
Scheme 11.1. Summarizing scheme of hydrogenation and related transformations catalyzed by rhenium(I) complexes, as well as alkali metal compounds.
197 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Chapter 1 of the thesis gives a general introduction and motivation for rhenium catalysis.
Following this, Chapter 2 consists of preparation of appropriate rhenium complexes and their application to olefin hydrogenations. Large bite angle diphosphine ligands A, B and C
(Figure 11.1) were used to prepare complexes of the type [Re(P∩P)(CH3CN)Br2(NO)] (P∩P
= Sixantphos (A), IIA; P∩P = Sixantphos-Ph2 (B), IIB; P∩P = Thixantphos (C), IIC), as well as the dinuclear complex [Re2(A)2(Br)2(µ-Br)2(NO)].2CH3CN (IIIA) were prepared. On
NO Ph2 P Br X O Re P NCCH3 Ph2 Br
IIA, X = Si(CH3)2 IIB, X = Si(Ph)2 IIC, X = S, S NO Br P Br P O O Re Re O . 2 CH3CN P Br P PPh2 PPh2 Br NO Thixantphos (C) IIIA
Figure 11.1. Ligands A-C and their nitrosyl rhenium complexes of the type II and III.
attempts to prepare complexes of the type [Re(P∩P)(ɳ2-ethylene)HBr(NO)] (VIA and VIB) by the reaction of IIA-IIB with Et3SiH and ethylene, a reaction channel was discovered revealing orthometallation of one of the phenyl groups of the diphosphine ligand (Figure
2 11.2). Thus, four membered rhenacycles of the type [Re(oCPPh-P∩P)(η -ethylene)Br(NO)]
(VIIA and VIIB) were formed. VIIA and VIIB were composed of the isomers VIIA1,
VIIA2 and VIIB, VIIB2, respectively (Figue 11.2). These isomers were separated by column chromatography. When these orthometallated complexes were reacted with H2, they underwent back reaction with hydrogenolysis of the Re-CPPh bond leading to the transient formation of the VIA and VIB which were highly reactive in hydrogenations of olefins generating the coordinatively unsaturated active species of type [Re(P∩P)HBr(NO)] (VA and
198 Summary
R R Si P O = O P PPh2 PPh2 A R = CH3 ( ) R = Ph (B)
Figure 11.2. Complexes of types V (the active species in hydrogenations of olefins and hydrosilylations of nitriles), VI and VII.
VB).
All the complexes of type II and IIIA, as well as of type VII were found to be active catalysts in the hydrogenation of olefins in the presence or absence of an activating co- catalyst. For instance, using complex IIIA along with 115 equiv. of Et3SiH as an activating
-1 co-catalyst TOFs up to 14000 h could be achieved under a H2 pressure of 10 bar at 120 °C in the hydrogenation of styrene. No loss of activity was observed in the hydrogenation of
199 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
R R'' R R'' Re(I) cat./Et3SiH (115-125 equiv.)
10-50 bar H , 80-140 oC R' H 2 R' H Toluene TOF: up to 14000 h-1 R, R', R'' =alkyl, aryl, C(O)OMe, CH C(O))Me TON: up to 31500 2
Scheme 11.2. Hydrogenation of olefins catalyzed by complexes of type II and IIIA, as well as of type VII.
styrene giving complete conversion to ethyl benzene in TONs of up to 29900.
The complexes IIB, VIIA and VIIB were also found to be efficient catalysts for this transformation, but were less active when compared to complex IIIA. Olefins pocessing β-
hydrogen atoms were found to be less active due to a prevailing isomerization reaction
leading to internal olefins. Apart from styrene, all the following listed olefins were
hydrogenated under different catalytic conditions using catalysts VIIA1 and VIIA2. A few
-1 selected reactions are given: 1-hexene (cat. VIIA1, 10 bar H2, 80 °C, TOF: 4867 h (TON at
-1 28% conv.)), cyclohexene (cat VIIA2, 10 bar H2, 120 °C, TOF: 1231 h ( TON at 37%
-1 conversion)), α-methyl styrene (cat VIIA2, 10 bar H2, 120 °C, TOF: 1940 h , TON: 20025,
-1 100%) and dimethyl itaconate (cat VIIA1, 10 bar H2, 140 °C, TOF: 500 h , TON: 31500,
95%)). The complex VIIA was tested also in the hydrogenation of an alkyne, phenylacetylene (50 bar H2, 140 °C) which showed complete conversion to ethyl benzene in
< 10 h when a loading of 0.043 mol% of it was adopted. A catalytic cycle operating for hydrogenation reactions of olefins using II, IIIA and VII is elucidated with the active species
V. The diiodo complex [Re(POP)(I)2(NO)] (XIIIA), where the O atom of Sixantphos ligand is coordinated trans to NO ligand, as well as [Re(D)(Br)2(CH3CN)2(NO)] (D =
DBFmonophos; XIVD) (Figure 11.3) were also prepared and studied their catalytic activity towards the hydrogenation of styrene. They furnished TOFs (first hour) of 8590 h-1, TON:
200 Summary
NO Ph Ph P X Re O Ph P X Ph O NO Ph P NCCH 2 3 O Re Ph2P H CCN Br D X = I, XIIIA 3 Br X= Br, XVIIIA Si XIVD
Figure 11.3. Rhenium complexes XIIIA, XVIIIA and XIVD as well as ligand D.
-1 29000 and 2710 h , TON: 21180, respectively, under a pressure of 10 bar of H2 at 120 °C.
Reaction of complex IIIA with n-Bu4NBr showed the formation of complex XVIIIA analogous to XIIIA. The opportunity of facile and easier access of the site trans to NO ligand by addition of n-Bu4NBr is utilized in many transformations.
Chapter 3 describes the hydrogenations of nitriles which were carried out under a pressure of 50 or 75 bar of H2 and at a temperature of 140 °C with a loading of 0.1-0.5 mol% of complexes IIIA, IIB, VIIA or VIIB along with the presence or absence of Et3SiH as a co- catalyst (Scheme 11.3). These reactions showed selectivity towards formation of secondary or tertiary amines depending on the structure of the nitrile. Selected reactions include the hydrogenation of benzonitrile (VIIA, 0.1 mol%, 50 bar, 140 °C, TOF: 250 h-1 at 50% conv.,
90%), 3-tolunitrile (IIIA, 0.5 mol%, 75 bar, 140 °C, TOF: 198 h-1, 99%),
RCH2N Imine R H2 (50-75 bar) 0.1 - 0.5 mol% Re(I) Cat. +
RCN (RCH2)NH o With or without Et3SiH (Cat.), 140 C R = Aryl, cy-Hexyl; R =Aryl,PhCH , 2 THF Selectivity upto 95% cy-Hexyl + (RCH ) N TOF: up to 250 h-1 2 3 R = PhCH2; Selectivity: 85%
Scheme 11.3. Hydrogenation of nitriles using complexe IIB, IIIA and of the type VII.
201 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
2-thienylcarbonitrile (VIIA, 0.5 mol%, 75 bar, 140 °C, TOF (first h): 162 h-1, 99%), and cyclohexanecarbonitrile (IIIA, 0.5 mol%, 50 bar, 140 °C, TOF: 198 h-1, 99%) all showed
selectivity towards the formation of secondary amines whereas phenyl acetonitrile showed
selectivity towards the formation of tertiary amine (VIIA, 0.5 mol%, 50 bar, 140 °C, TOF
(first h): 176 h-1, 99%).
In Chapter 4, the complexes IIB, IIIA and of type VII and XIIIA were applied for
the reductive aminations (Scheme 11.4) and hydrogenations of imines (Scheme 11.5).
Scheme 11.4. Reductive amination of aldehydes catalyzed by complexes IIB, IIIA and of the type VII.
With aniline as amine, a variety of aldehydes were reductively aminated to furnish the
desired secondary amines with good to excellent yields in most of the cases; selected
examples using complex IIIA as catalyst with the condition of hydrogenation and TOF of the
first hour include, benzaldehyde (0.05 mol%, 90 °C, TOF: 962 h-1, 95%); 4-
methoxybenzaldehyde (0.05 mol%, 90 °C, 1107 h-1, 96%); 3-nitrobenzaldehyde (0.3 mol%,
120 °C, TOF: 171 h-1, 94%), 4-chlorobenzaldehyde (0.15 mol%, 90 °C, TOF: 135 h-1, 94%),
1-naphthaldehyde (0.2 mol%, 90 °C, TOF: 84 h-1, 91%), 2-thienylcarboxaldehyde (0.05
mol%, 90 °C, TOF: 871 h-1, 97%), trans-cinnamaldehyde (0.1 mol%, 90 °C, TOF: 309 h-1,
66%). Reductive amination of benzaldehyde with 4-iodoaniline using 0.05 mol% of IIIA at
90 °C showed the desired product with a TOF of 1434 h-1 in the first hour, but prolonged run
of this reaction led to deiodinated product, N-benzylaniline in 90% yield. Under these
202 Summary
conditions, 4-nitroaniline showed a TOF of 376 h-1 in in the first hour giving rise to 97% yield of the desired product in 9 h. A concomitant hydrogeneation of formaldehyde to methanol was observed wherever it was used for reductive amination. Reductive amination of hexanal with 1-hexylamine (120 °C, TOF: 37 h-1 (first hour)) giving rise to a yield 76% of the desired product when a loading 0.2% of IIIA was adopted). Since formation of imines is not possible with secondary amines, their reductive alkylation gave lower yields due to the prevailing hydrogenation of the aldehydes to their corresponding alcohols.
In contrast to the reductive amination reactions, the hydrogenations of amines were highly efficient. The activity of the complex IIIA in the hydrogenations of various imines is as follows. N-benzylideneaniline (0.02 mol%, 90 °C, TOF (first 0.25 h): 3900 h-1, 97%); N-
(4-methoxybenzylidene)aniline (0.02 mol%, 90 °C, TOF (first 0.25 h): 11700 h-1, 97%);
Scheme 11.5. Hydogenation of imines catalyzed by IIIA and of the type VII.
N-(4-nitrobenzylidene)aniline (0.5 mol%, 120 °C, >192 h-1, 96%); N-(4- fluorobenzylidene)aniline (0.1 mol%, 90 °C, TOF (first 0.25 h): 2070 h-1, 99%); N-(4- chlorobenzylidene)aniline (0.1 mol%, 90 °C, TOF (first 0.25 h): 1012 h-1, 97%); N- benzylidene-4-nitroaniline (0.1 mol%, 90 °C, >330 h-1, 99%), N-benzylidene-4-fluoroaniline
(0.1 mol%, 90 °C, TOF (first 0.25 h): 2810 h-1, 99%); N-benzylidene-4-chloroaniline (0.1 mol%, 90 °C, TOF (first 0.25 h): 1555 h-1, 98%), N-benzylidene-4-methoxyaniline (0.02 mol%, 90 °C, TOF (first 0.25 h):: 7760 h-1, 98%, 98%), N-(4-methoxybenzylidene)-4- chloroaniline (0.02 mol%, 90 °C, >2425 h-1, 97%), N-benzylidene-1-naphthylamine (0.15
203 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
mol%, 120 °C, 950 h-1, 95%), N-benzylidene-1-hexylamine (0.1 mol%, 120 °C, 106 h-1,
95%), N-benzylidene-isobutylamine (0.15 mol%, 120 °C, 79 h-1, 95%), N-(1-
Phenylethylidene)aniline (0.5 mol%, 90 °C, > 198 h-1, 99%).
The reductive anilation of benzaldehyde using 0.02 mol% of IIIA in the presence of
100 equiv. of n-Bu4NBr with respect to IIIA, (which would form XVIIIA) was carried out at
90 °C showing a TOF of 8122 h-1 in the first 0.25 h, giving rise to a yield of 97% of the
desired product, N-benzylaniline in 97% yield within an hour. The reductive anilation of
benzaldehyde using 0.02 mol% of XIIIA under the conditions of 50 bar of H2 at 90 °C
showed a TOF of 2200 h-1 in the first 0.25 h giving rise to 97% yield of the desired product
N-benzylaniline within 4 h.
From mechanistic studies, a catalytic cycle with XXIA as the active species is
proposed for this hydrogenation of imines. It is formed by the dissociation of a bromide
ligand followed by isomerisation, leading to the accessibility of the site trans to NO ligand. A
P O = P
Figure 11.4. The active species XXIA in the hydrogenation of imines.
hydride ligand trans to NO ligand is much more hydridic in character and would eventually
lead to transfer of it to the polar substrates.
In Chapter 5, complex IIIA in the presence or absence of n-Bu4NBr (5 eqiuv. with
respect to IIIA) as co-catalyst was applied also for the hydrogenation of carbon dioxide
giving methanol in TONs of up to 88 at a pCO2 : pH2 = 20 : 60 at 140 °C run for 18 h
(Scheme 11.6). Complexes [Re(A)(Br)3(NO)][NEt4] (XIXA) and [Re2(A)2µ-(OH)3(NO)2]
204 Summary
(XXVA) were also prepared (Figure 11.6). These complexes were then applied in the hydrogenation of CO2 furnishing TON of 7 and 20 respectively under a pCO2 : pH2 = 10 : 30
[Re] CO2 + 3 H2 CH3OH + H2O With or without co-catalyst 10-20 bar o 30-60 bar THF, 140 C, TON up to 88 18-96 h Scheme 11.6. Hydrogenation of carbon dioxide to methanol catalyzed by complexes IIIA, XIIIA, XIXA and XXVA.
IIIA CH OSiEt CO2 + 3 Et3SiH 3 3 + (Et3Si)2O With or without co-catalyst 2-20 bar THF, 80-140 oC, TON up to 420 1-18 h Scheme 11.7. Hydrosilylation of carbon dioxide to methanol level using complexes IIIA or XVIIA.
Scheme 11.8. Combined hydrogenations/hydrosilylation of carbon dioxide to methanol level using complexes IIIA or XVIIA
+ NO ON H NO P O P Br- P Br Re Re O Re NEt O O O 4 P H P P Br O Br H XIXA XXVA
P O = P
Figure 11.6. Complexes XIXA and XXVA, and the active species XXVIIA in the transfer hydrogenation reactions of ketones, imines and nitriles as well as hydrogenation/hydrosilylation reactions of CO2 (right).
205 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
at 140 °C run for 18 h. The hydrosilylation of CO2 with Et3SiH (1000 equiv. with respect to
catalyst) furnished CH3OSiEt3 with TONs of up to 420. Also, a combined hydrogenation/hydrosilylation (pCO2 : pH2 = 20 : 60, 100 equiv. of Et3SiH) of carbon dioxide to methanol was also realized giving TONs up to 330 when run for 96 h at 140 °C (Scheme
11.7). The mechanism of these reactions of hydrogenation of CO2 is proposed to be operated through the active species XXVIIA (Figure 11.6).
In Chapter 6, the hydrogenation reaction has also been extended to aldehydes, ketones, esters and bicarbonates giving rise to the corresponding alcohols (Scheme 11.9).
-1 TOFs of up to 4500 h and TONs of up to 19200 with 99% yield were realized at 50 bar H2 pressure and at 140 °C for the hydrogenation of aldehydes. Selected examples of hydrogenation of aldehydes with catalyst loading, TOFs and yield are as follows: benzaldehyde (0.005 mol%, 2133 h-1, 96%);
Scheme 11.9. Hydrogenation of aldedhydes catalyzed by IIIA.
p-anisaldehyde (0.005 mol%, 2857 h-1, > 99%); 4-chlorobenzaldehyde (0.02, 1250 h-1, >
99%), 2-thiophenecarboxaldehyde (0.02 mol%, 1125 h-1, 95%); cyclohexanecarboxaldehyde
(0.02, 3920 h-1, 98%), hexanal (0.02, 4500 h-1, 90%); paraformaldehyde (0.05, 9 h-1, 11%); trans-cinnamaldehyde (0.05, 960 , 96%).
Under the same conditions, but with a loading of 0.05 mol% of IIIA, the hydrogenation reaction of ketones were also carried out (Scheme 11.10). As expected, the
206 Summary
Scheme 11.10. Hydrogenation of ketones catalyzed by IIIA.
efficiencies of TOF and yields of the tested ketones are as follows: acetophenone (167 h-1,
100%), benzophenone (50 h-1, 30%), 4ʹ-fluoroacetophenone (162 h-1, 97%), 2- acetylthiophene (23 h-1, 14%). The aliphatic ketone, cyclohexanone gave a yield of only <5% in 12 h due to a concomitant aldol reaction.
The hydrogenation of methyl formate to methanol could be realized with TON of 112 in THF when a catalyst loading of 0.5 mol% was adopted under a H2 pressure of 50 bar at
140 °C (Scheme 11.11). Benzyl benzoate with the addition of n-Bu4NBr furnished benzyl alcohol with a TON of 500. Under these conditions, but in EtOH, dimethyl carbonate provided a TON of 161. The hydrogenation of sodium bicarbonate to methanol in TON of 20 could be achieved for the first time when the reaction was carried out in EtOH in the presence of n-Bu4NBr as a co-catalyst. Also, this reaction furnished sodium formate in TON of 24.
IIIA (0.02-0.5 mol%) O With or without n-Bu NBr (5 equiv.) 4 R OH + R' OH R OR' H2 (50 bar) o 140 C, 15 h, Solvent Yield up to 100% R = H, Ph, OMe, OH TON > 500 R' = Me, PhCH2, Na
Scheme 11.11. Hydrogenation of esters and bicarbonates to alcohols using complex IIIA.
Using IIIA as a catalyst, the hydrogenations of CO2 to formate salts were also realized with 2,2,6,6-tetramethylpiperidine (TMP) or NaHCO3 as base under a relatively
207 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
lower temperature of 100 °C (Scheme 11.12). Overall TON of up to 605 could be realized
when sodium bicarbonate was used as a base in the presence of 5 equiv of n-Bu4NBr in
MeOH.
Scheme 11.12. Hydrogenation of carbon dioxide to formate salts using IIIA.
In Chapter 7, Claisen-Tishchenko disproportionative esterification of aldehydes and
transfer hydrogenations of ketones and imines were described. Highly efficient Claisen-
Tishchenko disproportionations of benzaldehyde to benzyl benzoate were realized with a
loading of 0.05 mol% of IIIA along with suitable silyl hydrides as a co-catalyst. Though an extensive optimization of this reaction showed TOFs of up to 3360 h-1 and yields up to 88%, concomitant hydrosilylations were always observed which retarded the reaction. However, the Claisen-Tishchenko reaction could be realized using complex IIIA along with LiAlH4 as a co-catalyst (Scheme 11.13). Complex IIIA along with 5 equiv. of LiAlH4 with respect to
IIIA, eventually generated the species XXVIIA. With a loading of 0.1 or 0.2 mol% of IIIA
along with 1 mol% of LiAlH4, though the reaction of aromatic aldehydes were carried out at
elevated temperatures of 80 °C or 110 °C, aliphatic aldehydes were smoothly converted to
Scheme 11.13. Claisen-Tishchenko reaction of aldehydes catalyzed by IIIA/LiAlH4 system.
208 Summary
their corresponding carboxylic esters at room temperature, furnishing yields of 90-98%.
However, the reaction of paraformaldehyde furnished a yield of only 24% of the desired ester
methylformate with a TOF of 120 h-1. For each substrates, the reaction temperature, TOFs
(first hour) and yield of the desired ester are as follows; benzaldehyde (80 °C, 260 h-1, 97%);
4-chlorobenzaldehyde (110 °C, 275 h-1, 96%); p-anisaldehyde (110 °C, 495 h-1, 99%); 2-
thienylcarboxaldehyde (110 °C, 391 h-1, 97%); 2-furancarboxaldehyde (110 °C, 150 h-1,
90%); hexanal (23 °C, 490 h-1, 98%); isobutyraldehyde (23 °C, 465 h-1, 93%);
cyclohexanecarboxaldehyde (23 °C,435 h-1, 96%); H (110 °C, 120 h-1, 24%).
Based on experimental studies, a mechanism involving the generation of rhenium
dihydrides XXVIIA followed by rhenium alkoxide as the active species is proposed.
Following this, highly efficient transfer hydrogenation reactions of ketones and
imines were also realized using 0.02-0.2 mol% of IIIA along with t-BuOK at 83 °C (Scheme
11.14 and Scheme 11.15). For instance, with a loading of 0.02 mol% of IIIA along with 1
mol% of t-BuOK at 83 °, acetophenone showed an initial TOF of 9200 h-1 with 46%
conversion in 0.25 h. Yields of up to 95% were realized in the transfer hydrogenations of
ketones. The activity of the complex IIIA towards this reaction is summarized as; ketone:
(TOF (first 0.25 h), yield); acetophenone: (9200 h-1, 89%); 4ʹ-methoxyacetophenone: (6191 h-
1, 66%); 4ʹ-methylacetophenone: (1529 h-1, 82%); 4ʹ-chloroacetophenone: (6090 h-1, 76%); 2- acetylthiophene: (3568 h-1, 66); benzophenone: (3554 h-1, 95%); 3,3'- bis(trifluoromethyl)benzophenone: (1061 h-1, >99%); Cyclohexanone: (2560 h-1, 99%);
Scheme 11.14. Transfer hydrogenation of ketones catalyzed by IIIA/t-BuOK/2-propanol system.
209 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
2-cyclohexenone: (2108 h-1, 74% (cyclohexanol as the product)).
Relatively higher loadings of 0.5 mol% of IIIA along with 1 mol% of t-BuOK were applied for the hydrogenation of aromatic aldimines giving up to 99% yield of the desired products (Scheme 11.14). This reaction was found to be less efficient for aliphatic imines.
However, imine bearing an aliphatic part, N-benzylidineisobutylamine were carried out with
1 mol% loading of IIIA giving a yield of 48% of the desired product in 5 h. The TOF (first h) and yield of the tested aromatic imines are as follows; N-benzylideneaniline: (136 h-1, 95%);
Scheme 15. Transfer hydrogenation of aldimines catalyzed by IIIA/t-BuOK/2-propanol system.
N-(4-methoxybenzylidene)aniline: (127 h-1, 89%); N-(4-fluorobenzylidene)aniline: (129 h-1,
98%); N-(4-methoxybenzylidene)-4-chloroaniline: (170 h-1, 99%); N-benzylidene-4- chloroaniline: (167 h-1, 95%).
Based on experiments and kinetic studies, a mechanism with XXVIIA as the active species is proposed for the transfer hydrogenation of ketones. A β-hydride abstraction to form
XXVIIA is suggested to be rate limiting. An analogous mechanism is also expected to be operative for the transfer hydrogenations of imines.
Following the studies of transfer hydrogenations of ketones and imines using IIIA/t-
BuOK/2-propanol system, this reaction has been then applied to nitriles (Scheme 11.16), described in Chapter 8. The transfer hydrogenations of aliphatic, aromatic and heteroaromatic nitriles with a temperature controlled selective formation of either primary amines and N-
210 Summary
isopropylideneamines (which can easily be hydrolyzed to primary amines) or reductively
alkylation products N-(isopropyl)benzylamine were also realized using complex IIIA/t-
BuOK/2-propanol system. With 1-2.5 mol% of IIIA along with 3 mol% of t-BuOK carrying
out at tempearatures of 83 °C furnished primary amines and isopropylideneamines
(RCH2N=(CH3)2 in good to excellent yields. This reaction when carried out at 140 °C gave
Scheme 11.16. Selective Transfer hydrogenation as well as reductive alkylation of nitriles to amines and N- alkylisopropylamine respectively using IIIA/t-BuOK/2-propanol system.
rise to the reductively alkylated products N-(isopropyl)amines
(RCH2NHCH(CH3)2respectively, in quantitative yields.
The TOFs for the formation of primary amine and N-isopropylideneamines
(RCH2N=C(CH3)2 together along with their yields in the transfer hydrogenation reactions of
the tested nitriles are as follows; Benzonitrile: (384 h-1, 93%); m-tolunitrile: (396 h-1, 99%); p-tolunitrile: (376 h-1, 94%); p-anisonitrile: (252 h-1 (first h), 83%); 2-bromobenzonitrile:
(392 h-1, 98%); 4-bromobenzonitrile (388 h-1, 97%); 3-trifluoromethylbenzonitrile: (396 h-1,
99%); 3-phenoxybenzonitrile: (396 h-1, 99%); 3,4-difluorobenzonitrile: (192 h-1, 96%); 2- thiophenecarbonitrile: 98 h-1 (first h), 90%; cyclohexanecarbonitrile: (120 (first h), 94%); phenylacetonitrile, (182 h-1 (first h), 95%).
211 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
For the reductively alkylation products 4, the yield are given as; R = Ph (93%), p- anisonitrile (69%), 3-trifluoromethylbenzonitrile (99%), 3-phenoxybenzonitrile (99%).
A mechanism analogous to those proposed for the transfer hydrogenations of ketones and imines is suggested for this transfer hydrogenations of nitriles with XXVIIA as the active species.
Efficient hydrosilylations of nitriles using Et3SiH was also described in Chapter 9, with a mechanism involning VA as the active species. TOFs of up to 98 h-1 were achieved
with almost equimolar ratios of nitriles and Et3SiH were subjected to hydrosilylation reaction
at 80 °C to furnish the corresponding N-silylaldimines up to 98%
Scheme 11.17. Hydrosilylation of nitriles and synthetic utility of the formed silylimines for the production of higher imines/amine protection.
Yield (Scheme 11.17). Aliphatic nitriles were much less efficient for this reaction even at a
temperature of 120 °C. For the first time, a synthetic utility of the N-silylaldimines were
explored in a one-pot fashion by the addition of amines to the formed N-silylaldimines and
that any nucleophilic addition to N-silylaldimines are scarce in literature.
The TOF and yield for the formation of N-silylimines (2) are as follows; benzonitrile:
(78 h-1, 98%); R = m-tolunitrile: (65 h-1, 98%); p-tolunitrile: (78 h-1, 98%); 2-
bromobenzonitrile: (65 h-1, 98%); 4-bromobenzonitrile: (32 h-1, 42%); 3-
trifluoromethylbenzonitrile: (4 h-1, 42%); 3,4-difluorobenzonitrile: (98 h-1, 98%); 3-chloro-4-
212 Summary
fluorobenzonitrile: (66 h-1, 95%); 2-thiophenecarbonitrile: (22 h-1, 88%); phenylacetonitrile:
(8 h-1, 33%).
Most of the transformations summarized above using rhenium complexes are either
comparable or even much more active in comparison to the corresponding transformations
reported using any homogeneous catalytic systems reported in literature.
Finally, simple and efficient method for the Claisen-Tishchenko disproportionation of
aldehydes to the corresponding carboxylic esters using alkali metal hydride, tert-butoxides and bis(trimethylsilyl)amides has been described (Scheme 11.18). When 18-crown-6 was added ad a co-catalyst to the reactions catalyzed by potassium derivatives, a drastic increase
Scheme 11.18. Claisen Tishchenko reaction of aldehydes catalyzed by sodium or potassium compounds.
in rate of reaction was observed in few cases showing TOFs of up to 1176 h-1 completing the
reaction even in 5 min with yields of up to 97%. Based on experimental studies, a mechanism
involving the metal alkoxide as active species is proposed for these reactions.
213 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
List of New Compounds
1. 4,6-Bis(diphenylphosphino)-10,10 diphenylphenoxasilin (Sixantphos-Ph2) (B)
2. [Re(A)(CH3CN)Br2(NO)], A = 4,6-Bis(diphenylphosphino)-10,10
dimethylphenoxasilin (Sixantphos) (IIA)
3. [Re(B)(CH3CN)Br2(NO)], C = 4,6-bis(diphenylphosphino)phenoxathiin
(Thixantphos) (IIC)
4. [Re2(A)2(Br)2(µ-Br)2(NO)].2CH3CN; (IIIA)
2 5. [Re(oCPPh-A)(η -ethylene)Br(NO)] (VIIA1)
2 6. [Re(oCPPh-A)(η -ethylene)Br(NO)] VIIA2
2 7. [Re(oCPPh-B)(η -ethylene)Br(NO)] (VIIB1)
2 8. [Re(oCPPh-B)(η -ethylene)Br(NO)] (VIIB1)
9. [Re(POP)I2(NO)] (XIIIA)
10. [Re(POP)Br2(NO)] (XVIIIA)
11. [Re(D)Br2(CH3CN)2(NO)], D = DBFmonophos (XIVD)
12. [Re(A)(NH3)Br2(NO)] (XVIA)
13. [Re(A)(PhCH=NH)Br2(NO)] (XVIIA)
14. [Re(A)Br3(NO)][NEt4] (XIXA)
15. [Re(A)Cl3(NO)][NEt4] (XXA)
16. [Re2(A)2µ-(OH)3(NO)2] (XXVIA)
214 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
Abstract
Homogeneous catalytic processes are often based on precious metals like rhodium, iridium, ruthenium, palladium and platinum. Rhenium being border to precious metals has preserved at least some of the precious metal characters. This thesis explored the ability of novel nitrosyl diphosphine rhenium(I) complexes for a variety of transformations like hydrogenations of alkenes and alkynes to alkanes, aldehydes, ketones and esters to alcohols, imines including direct reductive aminations and nitriles to amines, carbon dioxide and bicarbonates to methanol and formates. Also, transfer hydrogenations of ketones to alcohols, imines and nitriles to amines and hydrosilylations of nitriles to N-silylaldimines as well as
Claisen-Tishchenko disproportionation of aldehydes to esters were described. Most of these reactions are comparable and some are even much more active compared to the reported precious metal catalysis. Claisen-Tishchenko reaction of aldehydes to esters using alkali metal compounds was also realized.
215 Abstract
Zusammenfassung
Homogene katalytische Prozesse basieren oft auf edlen Metallen wie etwa Rhodium, Iridium,
Ruthenium, Palladium oder Platin. Rhenium, welches an der Grenze zu diesen Edelmetallen steht, verfügt zumindest teilweise über den benötigten Edelmetallcharakter. Die vorligende
Arbeit untersucht die Eignung neuer Nitrosyl Diphosphin Re(I) Komplexe für eine Vielzahl von Transformationen, zum Beispiel die Hydrierung von Alkinen und Alkenen zu Alkanen sowie von Aldehyden, Ketonen und Estern zu Alkoholen, Nitrilen und Iminen – einschliesslich reduktiver Aminierung – zu Aminen, Kohlenstoffdioxid und Bicarbonaten zu
Methanol und Formiaten. Zudem werden die Hydrierung von Ketonen zu Alkoholen, Iminen und Nitrilen zu Aminen und die Hydrosilylierung von Nitrilen zu N-Silylaldiminen sowie die
Claisen-Tishchenko Disproportionierung von Aldehyden zu Estern beschrieben. Die meisten der hier beschriebenen Reaktionen sind vergleichbar mit bekannten edelmetallkatalysierten
Umsetzungen oder übertreffen diese. Auch die Claisen-Tishchenko Reaktion von Aldehyden zu Estern unter Beteiligung von Alkalimetallverbindungen wurde realisiert.
216 Acknowledgements
Acknowledgements
I would like to thank several people who were involved in various ways during the accomplishment of my research work.
First of all, I would like to thank my supervisor Prof. Dr. Heinz Berke for giving me the opportunity to work in his laboratory and introducing me an interesting and challenging field of scientific research. I will remain indebted to him forever for lot of scientific discussions, ideas, guidance towards proper directions, encouragement and support.
Appreciation and thanks is also gratefully expressed to Prof. Dr. Roger Alberto for his kind acceptance to co-referee this dissertation.
I thank Dr. Olivier Blacque for solving lot of crystal structures during my research work.
I thank Dr. Thomas Fox for measuring some NMR samples and discussions regarding them.
I would like to extend my thanks to Heinz Spring and Barbara Spring for measuring all the elemental analyses.
Many thanks to Dr. Koushik Venkatesan for his enormous help and encouragement in various ways throughout my research work.
Thanks to Hanspeter Stadler for helping hand anytime for technical problems regarding glove box or gas room. I would really appreciate his innovative set up in the gas room.
Thanks to Dr. Balz Dudle for scientific discussions and suggestions during the initial part of my research work.
Thanks to Dr. Christian Frech for some scientific discussions.
I would like to express my gratitude to the administrative staff; Beatrice Spichtig, Susanna Sprokkereef, Nathalie Fichter, Dr. Jae Kuoung Pak, Ramona Erni, Manfred Jöhri and Dr. Ferdinand Wild.
217 Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes
I would like to thank my colleagues and friends for making nice working ambiences and extending helping hands in various ways during these years; Dr. Jai Anad Garg, Dr. Subrata Chakraborty, Dr. Samir Barman, Dr. Rajkumar Jana, Dr. Pandiarajan Devaraj, Dr. Yan Li, Dr. Carolina Egler, Dr. Yanfeng Jiang, Dr. Anne Landwehr, Gabriele Grieco, Michael Koch, Franziska Lissel, Pascal Plüss, Miriam Oberholzer, Alexander Szentkuti and Michel Bachmann. Thanks a lot to all of you for making my stay in Zürich memorable.
I thank all the members of Inorganic Chemistry Institute.
I thank the University of Zurich, Swiss National Science Foundation and Lanxess AG (Leverkusen, Germany) for financial support.
Thanks to Prof. Dr. K. K. Balasubramanian and Dr. R Saiganesh and M. Somasundaram for their scientific assistance during my stay in Shasun Research Centre, Chennai.
Thanks to Prof. Dr. Irishi N. N. Namboothiri for his scientific guidance during my stay in Indian Institute of Technology Bombay.
I also thank all my teachers as well as all those inculcate in me several good qualities.
Thanks to my native friends Binil P. Sasidharan and Jayachandran V for their help.
Finally my special thanks to my father N. Kunjanpillai (Late), mother Kamalakshiamma, wife Sujatha Rajesh, daughter Kavyalakshmi R, brothers K. Kuttanpillai, K. Sasi Kumar and K. Santhosh Kumar, sister-in-laws Bindhu K. Pillai, Geetha S. Pillai and Rani S. Pillai, my niece Kavitha K, nephews Kannan K, Nandu S. Pillai, Sabarinath S. Pillai and Sathamanyu S. Pillai.
218
Curriculum Vitae
Name: RAJESH KUNJANPILLAI
Date of Birth: 15th October 1980
Gender: Male
Nationality: Indian
Martial Status: Married to Sujatha Pullakhandam
Children: Kavyalakshmi R (Daughter)
Current Position: PhD Student (01st June 2009 – till date) University of Zürich, Switzerland. Title of the Thesis: Homogeneous Hydrogenations and Related Reductive Reactions Catalyzed by Rhenium Complexes Advisor: Prof. Dr. Heinz Berke
Education
University / Institute Degree Year University of Zürich, Zürich, PhD (Pursuing) 2009-2013 Switzerland
Pondicherry University, Master’s Degree in Chemical 2003-2005 Pondicherry, India Sciences
M.G.University College of Teacher Bachelor’s Degree in 2001-2002 Education, Pathanamthitta, India Education
St. Stephen’s College, Bachelor’s Degree in 1998-2001 Pathanapuram, India Chemistry
219 Professional Experience
Academic
Institute/Industry Designation Duration
Department of Chemistry, Indian 18th Sept. 2008-08th May 2009 Institute of Technology Bombay, Senior Research Fellow Mumbai
Industry
Research Associate Shasun Research Centre, Chennai 14th July 2005-30th Aug. 2008 (R & D Officer)
Academic Achievements
Passed UGC-CSIR-National Eligibility Test for Lectureship, June 2006, conducted jointly by University Grants Commission (UGC) and Council of Scientific and Industrial Research (CSIR), India.
Awards and Honors
Swiss Academy of Sciences/Swiss Chemical Society (SCNAT/SCS) Chemistry Travel Award 2012.
Research Interest
Organometallic Chemistry: Structure, reactivity and mechanistic studies, Organometallic homogeneous catalysis including asymmetric versions, Organic transformations using organometallic complexes or catalysts.
Synthetic Organic Chemistry: Carbon-Carbon and Carbon-Heteroatom (N, O, S, halogen) bond forming reactions, Reactions involving hydride transfer, Organic methodology, Organosilicon chemistry, Stereoselective transformations etc.
220 Scientific Research Publications
Published
6. Alkali Metal t-Butoxides, Hydrides and Bis(trimethylsilyl)amides as Efficient Homogeneous Catalysts for Claisen-Tishchenko Reaction. Kunjanpillai Rajesh, Heinz Berke, Adv. Synth. Catal. 2013, 355, 901-906.
5. Homogeneous Hydrogenations of Nitriles catalyzed by Rhenium Complexes. Kunjanpillai Rajesh, Balz Dudle, Olivier Blacque, Heiz Berke, Adv. Synth. Catal. 2011, 353, 1479-1484.
4. Rhenium in Homogeneous Catalysis: [ReBrH(NO)(labile ligand)(large-bite-angle diphosphine)] Complexes as Highly Active Catalysts in Olefin Hydrogenations. Balz. Dudle, Kunjanpillai Rajesh, Olivier Blacque, Heinz Berke, J. Am. Chem. Soc. 2011, 133, 8168-8178.
3. Rhenium Nitrosyl Complexes Bearing Large-Bite-Angle Diphosphines. Balz Dudle, Kunjanpillai Rajesh, Olivier Blacque, Heinz Berke, Organometallics 2011, 30, 2986-2992.
2. One-pot three component α-aminoalkylation of conjugated nitroalkenes and nitrodienes. Kunjanpillai Rajesh, Pramod Shanbhag, Manjoji Raghavendra, Pallavi Bhardwaj, Irishi N. N. Namboothiri, Tetrahedron. Lett. 2010, 51, 846-849.
1. Bromination of Deactivated Aromatics - A Simple and Efficient Method. K. Rajesh, M. Somasundaram, R. Saiganesh, K. K. Balasubramanian, J. Org. Chem, 2007, 72, 5867-5869.
Unpublished
7. Highly Efficient Homogeneous Catalysis of Direct Reductive Amination of Aldehydes and Hydrogenation of Imines Applying Rhenium Complexes. Kunjanpillai Rajesh, Olivier Blacque and Heinz Berke.
8. Homogeneous Hydrogenation/Hydrosilylation of Carbon Dioxide to Methanol Catalyzed by Rhenium Complexes. Kunjanpillai Rajesh, Olivier Blacque, Thomas Fox and Heinz Berke.
221
9. Homogeneous Hydrogenations of Aldehydes, Ketones, Esters and Bicarbonates to Alcohols, as well as Carbon Dioxide and Bicarbonates to Formates Catalyzed by Rhenium Complexes. Kunjanpillai Rajesh and Heinz Berke.
10. Homogeneous Claisen-Tishchenko Reactions of Aldehydes and Transfer Hydrogenation Reactions of Ketones and Imines Catalyzed by Rhenium Complexes. Kunjanpillai Rajesh and Heinz Berke.
11. Homogeneous Thermocontrolled Chemoselective Transfer Hydrogenations of Nitriles Catalyzed by Rhenium Complexes. Kunjanpillai Rajesh and Heinz Berke.
12. Homogeneous Hydrosilylations of Nitriles Catalyzed by Rhenium Complexes. Kunjanpillai Rajesh and Heinz Berke.
Conferences and Seminars
Kunjanpillai Rajesh, Balz Dudle, Olivier Blacque, Thomas Fox, Heinz Berke, Rhenium- Catalyzed Homogeneous Hydrogenations; XXV International Conference on Organometallic Chemistry, Sep. 2-7, 2012 – Lisbon, Portugal. (Oral Flash & Poster).
Balz Dudle, Kunjanpillai Rajesh, Olivier Blacque, Heinz Berke, Rhenium-Catalyzed Homogeneous Hydrogenations, Swiss Chemical Society Fall Meeting 2011, 09.09.2011, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne. (Poster).
Developing Rhenium Catalysts for Homogeneous Hydrogenation Reactions, Swiss-German Project; Unconventional Approaches to the Reactivity of Dihydrogen, 11.02.2011, Swiss Federal Institute of Technology (ETH) Zurich, Zurich, Switzerland. (Oral).
Permanent Address: Pullam Madathil Kaithaparampu Post Enathu Pathanamthitta District Kerala State - 691526 India
222